Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 3,289,242	$ 4,560,889
Gross Unrealized Losses, Less than 12 Months	30,440	68,703
Fair Value, 12 Months or Greater	613,124	599,999
Gross Unrealized Losses, 12 Months or Greater	15,441	30,565
Fair Value, Total	3,902,366	5,160,888
Gross Unrealized Losses, Total	45,881	99,268
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	3,284,564	4,560,553
Gross Unrealized Losses, Less than 12 Months	27,345	68,681
Fair Value, 12 Months or Greater	579,780	569,210
Gross Unrealized Losses, 12 Months or Greater	13,332	25,902
Fair Value, Total	3,864,344	5,129,763
Gross Unrealized Losses, Total	40,677	94,583
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	118,066	112,709
Gross Unrealized Losses, Less than 12 Months	1,048	1,252
Fair Value, 12 Months or Greater	48,880	35,450
Gross Unrealized Losses, 12 Months or Greater	1,076	1,341
Fair Value, Total	166,946	148,159
Gross Unrealized Losses, Total	2,124	2,593
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	650,353	1,562,614
Gross Unrealized Losses, Less than 12 Months	5,797	35,553
Fair Value, 12 Months or Greater	119,087	133,034
Gross Unrealized Losses, 12 Months or Greater	1,440	4,885
Fair Value, Total	769,440	1,695,648
Gross Unrealized Losses, Total	7,237	40,438
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	527,034	625,903
Gross Unrealized Losses, Less than 12 Months	5,000	11,103
Fair Value, 12 Months or Greater	221,923	109,066
Gross Unrealized Losses, 12 Months or Greater	5,310	4,828
Fair Value, Total	748,957	734,969
Gross Unrealized Losses, Total	10,310	15,931
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,116,878	1,010,836
Gross Unrealized Losses, Less than 12 Months	8,033	5,340
Fair Value, 12 Months or Greater	130,734	201,693
Gross Unrealized Losses, 12 Months or Greater	2,372	6,601
Fair Value, Total	1,247,612	1,212,529
Gross Unrealized Losses, Total	10,405	11,941
Corporate
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	651,373	1,035,245
Gross Unrealized Losses, Less than 12 Months	5,395	13,448
Fair Value, 12 Months or Greater	57,557	65,147
Gross Unrealized Losses, 12 Months or Greater	3,118	7,470
Fair Value, Total	708,930	1,100,392
Gross Unrealized Losses, Total	8,513	20,918
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	220,860	213,246
Gross Unrealized Losses, Less than 12 Months	2,072	1,985
Fair Value, 12 Months or Greater	1,599	24,820
Gross Unrealized Losses, 12 Months or Greater	16	777
Fair Value, Total	222,459	238,066
Gross Unrealized Losses, Total	2,088	2,762
Equity securities available for sale
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	4,678	336
Gross Unrealized Losses, Less than 12 Months	3,095	22
Fair Value, 12 Months or Greater	33,344	30,789
Gross Unrealized Losses, 12 Months or Greater	2,109	4,663
Fair Value, Total	38,022	31,125
Gross Unrealized Losses, Total	5,204	4,685
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	4,678	336
Gross Unrealized Losses, Less than 12 Months	3,095	22
Fair Value, 12 Months or Greater	9,387	8,755
Gross Unrealized Losses, 12 Months or Greater	391	1,024
Fair Value, Total	14,065	9,091
Gross Unrealized Losses, Total	3,486	1,046
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	0	0
Gross Unrealized Losses, Less than 12 Months	0	0
Fair Value, 12 Months or Greater	23,957	22,034
Gross Unrealized Losses, 12 Months or Greater	1,718	3,639
Fair Value, Total	23,957	22,034
Gross Unrealized Losses, Total	$ 1,718	$ 3,639