Investments (Amortized Cost and Fair Value of Fixed Maturity Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, cost or amortized cost	$ 424,041
Due after one year through five years, cost or amortized cost	1,671,193
Due after five years through ten years, cost or amortized cost	2,118,236
Due after ten years, cost or amortized cost	3,570,691
Total, cost or amortized cost	7,784,161
Total cost or amortized cost	8,709,444	$ 9,470,795
Due in one year or less, fair value	429,125
Due after one year through five years, fair value	1,757,547
Due after five years through ten years, fair value	2,241,236
Due after ten years, fair value	4,320,073
Total, fair value	8,747,981
Total fair value	9,744,199	$ 10,215,328
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	43,858
Fair value	44,638
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	878,548
Fair value	947,679
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	2,877
Fair value	$ 3,901