Investments (AFS Fixed Income securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less fair value	$ 331,997
Due after one year through five years fair value	1,989,887
Due after five years through 10 years fair value	2,294,091
Due after 10 years fair value	251,030
Total AFS fixed income securities	$ 4,867,005	$ 4,792,540