Securities In An Unrealized Loss Position Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 0	$ (18,114)
Fair Value, Less than 12 Months	4,031,893		$ 4,560,889
Gross Unrealized Losses, Less than 12 Months	48,808		68,703
Fair Value, 12 Months or Greater	518,319		599,999
Gross Unrealized Losses, 12 Months or Greater	24,738		30,565
Fair Value, Total	4,550,212		5,160,888
Gross Unrealized Losses, Total	73,546		99,268
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	95,253		112,709
Gross Unrealized Losses, Less than 12 Months	954		1,252
Fair Value, 12 Months or Greater	33,071		35,450
Gross Unrealized Losses, 12 Months or Greater	1,266		1,341
Fair Value, Total	128,324		148,159
Gross Unrealized Losses, Total	2,220		2,593
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,111,978		1,562,614
Gross Unrealized Losses, Less than 12 Months	22,836		35,553
Fair Value, 12 Months or Greater	150,439		133,034
Gross Unrealized Losses, 12 Months or Greater	3,733		4,885
Fair Value, Total	1,262,417		1,695,648
Gross Unrealized Losses, Total	26,569		40,438
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	618,642		625,903
Gross Unrealized Losses, Less than 12 Months	10,013		11,103
Fair Value, 12 Months or Greater	140,791		109,066
Gross Unrealized Losses, 12 Months or Greater	5,143		4,828
Fair Value, Total	759,433		734,969
Gross Unrealized Losses, Total	15,156		15,931
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,204,310		1,010,836
Gross Unrealized Losses, Less than 12 Months	4,386		5,340
Fair Value, 12 Months or Greater	95,421		201,693
Gross Unrealized Losses, 12 Months or Greater	5,094		6,601
Fair Value, Total	1,299,731		1,212,529
Gross Unrealized Losses, Total	9,480		11,941
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	867,293		1,035,245
Gross Unrealized Losses, Less than 12 Months	8,126		13,448
Fair Value, 12 Months or Greater	61,732		65,147
Gross Unrealized Losses, 12 Months or Greater	6,164		7,470
Fair Value, Total	929,025		1,100,392
Gross Unrealized Losses, Total	14,290		20,918
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	115,542		213,246
Gross Unrealized Losses, Less than 12 Months	1,320		1,985
Fair Value, 12 Months or Greater	4,495		24,820
Gross Unrealized Losses, 12 Months or Greater	257		777
Fair Value, Total	120,037		238,066
Gross Unrealized Losses, Total	1,577		2,762
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	4,013,018		4,560,553
Gross Unrealized Losses, Less than 12 Months	47,635		68,681
Fair Value, 12 Months or Greater	485,949		569,210
Gross Unrealized Losses, 12 Months or Greater	21,657		25,902
Fair Value, Total	4,498,967		5,129,763
Gross Unrealized Losses, Total	69,292		94,583
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	23,567		22,034
Gross Unrealized Losses, 12 Months or Greater	2,106		3,639
Fair Value, Total	23,600		22,034
Gross Unrealized Losses, Total	2,106		3,639
Equity securities available for sale
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	18,875		336
Gross Unrealized Losses, Less than 12 Months	1,173		22
Fair Value, 12 Months or Greater	32,370		30,789
Gross Unrealized Losses, 12 Months or Greater	3,081		4,663
Fair Value, Total	51,245		31,125
Gross Unrealized Losses, Total	4,254		4,685
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	18,875		336
Gross Unrealized Losses, Less than 12 Months	1,173		22
Fair Value, 12 Months or Greater	8,803		8,755
Gross Unrealized Losses, 12 Months or Greater	975		1,024
Fair Value, Total	27,678		9,091
Gross Unrealized Losses, Total	2,148		$ 1,046
Noninvestment Grade Investments at Loss Less than 5 Million [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	106,289
Noninvestment Grade Investments at Loss Less than 5 Million [Member] | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	19,647
Noninvestment Grade Investments at Loss Less than 5 Million [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	$ 19,990
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount		$ 18,100