Investments (Amortized Cost and Fair Value of Debt Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt Securities Available for Sale, Amortized Cost
Due in 1 year or less	$ 131,110
Due after 1 year through 5 years	830,945
Due after 5 years through 10 years	1,774,539
Due after 10 years	60,641
Amortized Cost, Single Maturity Date Securities	2,797,235
Mortgage and asset-backed securities	36,463
Total	2,833,698
Debt Securities Available for Sale, Fair Value
Due in 1 year or less	132,759
Due after 1 year through 5 years	878,847
Due after 5 years through 10 years	1,749,470
Due after 10 years	60,580
Fair Value, Single Maturity Date Securities	2,821,656
Mortgage and asset-backed securities	39,566
Total	2,861,222
Debt Securities Held to Maturity, Amortized Cost
Due in 1 year or less	152,369
Due after 1 year through 5 years	1,534,855
Due after 5 years through 10 years	3,663,415
Due after 10 years	320,307
Amortized Cost, Single Maturity Date Securities	5,670,946
Mortgage and asset-backed securities	1,503,021
Totals	7,173,967	$ 6,841,543
Debt Securities Held to Maturity, Fair Value
Due in 1 year or less	155,910
Due after 1 year through 5 years	1,641,932
Due after 5 years through 10 years	3,650,811
Due after 10 years	330,976
Fair Value, Single Maturity Date Securities	5,779,629
Mortgage and asset-backed securities	1,555,807
Total	$ 7,335,436	$ 7,175,443