Annual report pursuant to Section 13 and 15(d)

Variable Interest Entities (Narrative) (Details)

v3.8.0.1
Variable Interest Entities (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Variable Interest Entities [Abstract]      
Voluntary repurchases and substitutions $ 9.5 $ 6.5 $ 3.3