Investments (Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Debt Securities Available for Sale- Amortized Cost
Due in 1 year or less	$ 104,883
Due after 1 year through 5 years	854,799
Due after 5 years through 10 years	1,757,807
Due after 10 years	41,377
Amortized Cost	2,758,866
Mortgage and asset-backed securities	47,587
Total	2,806,453	$ 2,833,698
Debt Securities Available for Sale- Fair Value
Due in 1 year or less	107,348
Due after 1 year through 5 years	915,279
Due after 5 years through 10 years	1,789,908
Due after 10 years	42,293
Fair Value	2,854,828
Mortgage and asset-backed securities	51,082
Total	2,905,910	2,861,222
Debt Securities Held to Maturity - Amortized Costs
Due in 1 year or less	197,771
Due after 1 year through 5 years	1,682,002
Due after 5 years through 10 years	3,537,404
Due after 10 years	289,477
Amortized Cost	5,706,654
Mortgage and asset-backed securities	1,488,579
Total	7,195,233	7,173,967
Debt Securities Held to Maturity - Fair Value
Due in 1 year or less	200,277
Due after 1 year through 5 years	1,819,497
Due after 5 years through 10 years	3,626,374
Due after 10 years	303,922
Fair Value	5,950,070
Mortgage and asset-backed securities	1,575,795
Total	$ 7,525,865	$ 7,335,436	$ 7,335,436