Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 3,737,748	$ 4,560,889
Gross Unrealized Losses, Less than 12 Months	35,938	68,703
Fair Value, 12 Months or Greater	424,382	599,999
Gross Unrealized Losses, 12 Months or Greater	15,891	30,565
Fair Value, Total	4,162,130	5,160,888
Gross Unrealized Losses, Total	51,829	99,268
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	3,713,744	4,560,553
Gross Unrealized Losses, Less than 12 Months	30,985	68,681
Fair Value, 12 Months or Greater	390,753	569,210
Gross Unrealized Losses, 12 Months or Greater	14,069	25,902
Fair Value, Total	4,104,497	5,129,763
Gross Unrealized Losses, Total	45,054	94,583
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	115,584	112,709
Gross Unrealized Losses, Less than 12 Months	935	1,252
Fair Value, 12 Months or Greater	30,925	35,450
Gross Unrealized Losses, 12 Months or Greater	1,200	1,341
Fair Value, Total	146,509	148,159
Gross Unrealized Losses, Total	2,135	2,593
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	860,362	1,562,614
Gross Unrealized Losses, Less than 12 Months	10,961	35,553
Fair Value, 12 Months or Greater	112,597	133,034
Gross Unrealized Losses, 12 Months or Greater	1,468	4,885
Fair Value, Total	972,959	1,695,648
Gross Unrealized Losses, Total	12,429	40,438
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	585,106	625,903
Gross Unrealized Losses, Less than 12 Months	6,827	11,103
Fair Value, 12 Months or Greater	128,391	109,066
Gross Unrealized Losses, 12 Months or Greater	4,038	4,828
Fair Value, Total	713,497	734,969
Gross Unrealized Losses, Total	10,865	15,931
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,424,602	1,010,836
Gross Unrealized Losses, Less than 12 Months	6,921	5,340
Fair Value, 12 Months or Greater	75,179	201,693
Gross Unrealized Losses, 12 Months or Greater	3,692	6,601
Fair Value, Total	1,499,781	1,212,529
Gross Unrealized Losses, Total	10,613	11,941
Corporate
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	612,215	1,035,245
Gross Unrealized Losses, Less than 12 Months	4,599	13,448
Fair Value, 12 Months or Greater	41,688	65,147
Gross Unrealized Losses, 12 Months or Greater	3,641	7,470
Fair Value, Total	653,903	1,100,392
Gross Unrealized Losses, Total	8,240	20,918
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	115,875	213,246
Gross Unrealized Losses, Less than 12 Months	742	1,985
Fair Value, 12 Months or Greater	1,973	24,820
Gross Unrealized Losses, 12 Months or Greater	30	777
Fair Value, Total	117,848	238,066
Gross Unrealized Losses, Total	772	2,762
Equity securities available for sale
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	24,004	336
Gross Unrealized Losses, Less than 12 Months	4,953	22
Fair Value, 12 Months or Greater	33,629	30,789
Gross Unrealized Losses, 12 Months or Greater	1,822	4,663
Fair Value, Total	57,633	31,125
Gross Unrealized Losses, Total	6,775	4,685
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	24,004	336
Gross Unrealized Losses, Less than 12 Months	4,953	22
Fair Value, 12 Months or Greater	9,040	8,755
Gross Unrealized Losses, 12 Months or Greater	738	1,024
Fair Value, Total	33,044	9,091
Gross Unrealized Losses, Total	5,691	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	24,589	22,034
Gross Unrealized Losses, 12 Months or Greater	1,084	3,639
Fair Value, Total	24,589	22,034
Gross Unrealized Losses, Total	$ 1,084	$ 3,639