Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 5,809,019	$ 3,783,769
Gross Unrealized Losses, Less than 12 Months	92,122	31,682
Fair Value, 12 Months or Greater	1,091,418	1,155,860
Gross Unrealized Losses, 12 Months or Greater	37,924	28,547
Fair Value, Total	6,900,437	4,939,629
Gross Unrealized Losses, Total	130,046	60,229
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	138,917	92,167
Gross Unrealized Losses, Less than 12 Months	2,978	1,491
Fair Value, 12 Months or Greater	71,224	72,055
Gross Unrealized Losses, 12 Months or Greater	3,135	2,341
Fair Value, Total	210,141	164,222
Gross Unrealized Losses, Total	6,113	3,832
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,514,028	735,972
Gross Unrealized Losses, Less than 12 Months	20,922	5,944
Fair Value, 12 Months or Greater	303,246	345,755
Gross Unrealized Losses, 12 Months or Greater	9,269	6,630
Fair Value, Total	1,817,274	1,081,727
Gross Unrealized Losses, Total	30,191	12,574
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	754,800	480,435
Gross Unrealized Losses, Less than 12 Months	12,603	5,110
Fair Value, 12 Months or Greater	362,331	373,956
Gross Unrealized Losses, 12 Months or Greater	14,294	11,065
Fair Value, Total	1,117,131	854,391
Gross Unrealized Losses, Total	26,897	16,175
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,271,743	1,127,309
Gross Unrealized Losses, Less than 12 Months	9,444	8,298
Fair Value, 12 Months or Greater	160,263	167,412
Gross Unrealized Losses, 12 Months or Greater	2,196	2,314
Fair Value, Total	1,432,006	1,294,721
Gross Unrealized Losses, Total	11,640	10,612
Corporate
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,912,850	1,103,747
Gross Unrealized Losses, Less than 12 Months	42,874	8,224
Fair Value, 12 Months or Greater	168,813	170,858
Gross Unrealized Losses, 12 Months or Greater	8,753	5,946
Fair Value, Total	2,081,663	1,274,605
Gross Unrealized Losses, Total	51,627	14,170
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	216,681	244,139
Gross Unrealized Losses, Less than 12 Months	3,301	2,615
Fair Value, 12 Months or Greater	25,541	25,824
Gross Unrealized Losses, 12 Months or Greater	277	251
Fair Value, Total	242,222	269,963
Gross Unrealized Losses, Total	$ 3,578	$ 2,866