Investments (Fixed Income Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Fixed Income Securities [Line Items]
Due in one year or less fair value, AFS	$ 337,546
Due after one year through five years fair value, AFS	2,148,949
Due after five years through 10 years fair value, AFS	2,377,658
Due after 10 years fair value, AFS	269,279
AFS, Fair Value	5,133,432	$ 4,792,540
Due in one year or less carrying value, HTM	27,335
Due after one year through five years carrying value, HTM	29,341
Due after five years through 10 years carrying value, HTM	8,313
Due after 10 years carrying value, HTM	0
HTM, Carrying Value	64,989	101,556
Due in one year or less fair value, HTM	27,531
Due after one year through five years fair value, HTM	31,037
Due after five years through 10 years fair value, HTM	9,148
Due after 10 years fair value, HTM	0
HTM, Fair Value	$ 67,716	$ 105,211