Investments (Fixed Income Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Fixed Income Securities [Line Items]
Due in one year or less fair value, AFS	$ 231,898
Due after one year through five years fair value, AFS	1,931,022
Due after five years through 10 years fair value, AFS	2,742,707
Due after 10 years fair value, AFS	236,010
AFS, Fair Value	5,141,637	$ 5,162,522
Due in one year or less carrying value, HTM	10,888
Due after one year through five years carrying value, HTM	27,710
Due after five years through 10 years carrying value, HTM	2,989
Due after 10 years carrying value, HTM	0
HTM, Carrying Value	41,587	42,129
Due in one year or less fair value, HTM	10,979
Due after one year through five years fair value, HTM	29,226
Due after five years through 10 years fair value, HTM	3,058
Due after 10 years fair value, HTM	0
HTM, Fair Value	$ 43,263	$ 44,100