Securities In An Unrealized Loss Position Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 2,016,320	$ 3,497,447
Gross Unrealized Losses, Less than 12 Months	53,400	78,466
Fair Value, 12 Months or Greater	741,468	672,929
Gross Unrealized Losses, 12 Months or Greater	35,694	35,482
Fair Value, Total	2,757,788	4,170,376
Gross Unrealized Losses, Total	89,094	113,948
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	24,911	101,660
Gross Unrealized Losses, Less than 12 Months	79	487
Fair Value, 12 Months or Greater	43,226	64,500
Gross Unrealized Losses, 12 Months or Greater	902	1,846
Fair Value, Total	68,137	166,160
Gross Unrealized Losses, Total	981	2,333
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	210,419	501,952
Gross Unrealized Losses, Less than 12 Months	1,903	4,404
Fair Value, 12 Months or Greater	171,433	106,681
Gross Unrealized Losses, 12 Months or Greater	2,596	1,865
Fair Value, Total	381,852	608,633
Gross Unrealized Losses, Total	4,499	6,269
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	135,022	381,986
Gross Unrealized Losses, Less than 12 Months	1,748	3,639
Fair Value, 12 Months or Greater	155,445	184,807
Gross Unrealized Losses, 12 Months or Greater	4,416	7,669
Fair Value, Total	290,467	566,793
Gross Unrealized Losses, Total	6,164	11,308
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,207,153	1,091,078
Gross Unrealized Losses, Less than 12 Months	14,032	7,703
Fair Value, 12 Months or Greater	230,979	190,467
Gross Unrealized Losses, 12 Months or Greater	8,814	6,711
Fair Value, Total	1,438,132	1,281,545
Gross Unrealized Losses, Total	22,846	14,414
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	362,296	1,232,940
Gross Unrealized Losses, Less than 12 Months	25,162	35,406
Fair Value, 12 Months or Greater	79,582	76,797
Gross Unrealized Losses, 12 Months or Greater	10,998	8,987
Fair Value, Total	441,878	1,309,737
Gross Unrealized Losses, Total	36,160	44,393
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	41,278	169,190
Gross Unrealized Losses, Less than 12 Months	3,964	8,822
Fair Value, 12 Months or Greater	25,672	19,528
Gross Unrealized Losses, 12 Months or Greater	2,497	3,867
Fair Value, Total	66,950	188,718
Gross Unrealized Losses, Total	6,461	12,689
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,981,079	3,478,806
Gross Unrealized Losses, Less than 12 Months	46,888	60,461
Fair Value, 12 Months or Greater	706,337	642,780
Gross Unrealized Losses, 12 Months or Greater	30,223	30,945
Fair Value, Total	2,687,416	4,121,586
Gross Unrealized Losses, Total	77,111	91,406
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	35,241	0
Gross Unrealized Losses, Less than 12 Months	6,512	0
Fair Value, 12 Months or Greater	27,657	22,320
Gross Unrealized Losses, 12 Months or Greater	3,932	3,353
Fair Value, Total	62,898	22,320
Gross Unrealized Losses, Total	10,444	3,353
Equity securities available for sale
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	35,241	18,641
Gross Unrealized Losses, Less than 12 Months	6,512	18,005
Fair Value, 12 Months or Greater	35,131	30,149
Gross Unrealized Losses, 12 Months or Greater	5,471	4,537
Fair Value, Total	70,372	48,790
Gross Unrealized Losses, Total	11,983	22,542
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	0	18,641
Gross Unrealized Losses, Less than 12 Months	0	18,005
Fair Value, 12 Months or Greater	7,474	7,829
Gross Unrealized Losses, 12 Months or Greater	1,539	1,184
Fair Value, Total	7,474	26,470
Gross Unrealized Losses, Total	1,539	$ 19,189
Noninvestment Grade Investments at Loss Less than 5 Million [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	176,942
Noninvestment Grade Investments at Loss Less than 5 Million [Member] | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	35,540
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	$ 92,666