Securities In An Unrealized Loss Position Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 1,897,137	$ 3,497,447
Gross Unrealized Losses, Less than 12 Months	20,713	78,466
Fair Value, 12 Months or Greater	659,502	672,929
Gross Unrealized Losses, 12 Months or Greater	24,485	35,482
Fair Value, Total	2,556,639	4,170,376
Gross Unrealized Losses, Total	45,198	113,948
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	54,550	101,660
Gross Unrealized Losses, Less than 12 Months	279	487
Fair Value, 12 Months or Greater	39,092	64,500
Gross Unrealized Losses, 12 Months or Greater	652	1,846
Fair Value, Total	93,642	166,160
Gross Unrealized Losses, Total	931	2,333
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	217,215	501,952
Gross Unrealized Losses, Less than 12 Months	1,095	4,404
Fair Value, 12 Months or Greater	138,082	106,681
Gross Unrealized Losses, 12 Months or Greater	1,490	1,865
Fair Value, Total	355,297	608,633
Gross Unrealized Losses, Total	2,585	6,269
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	251,969	381,986
Gross Unrealized Losses, Less than 12 Months	1,851	3,639
Fair Value, 12 Months or Greater	105,927	184,807
Gross Unrealized Losses, 12 Months or Greater	3,924	7,669
Fair Value, Total	357,896	566,793
Gross Unrealized Losses, Total	5,775	11,308
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	918,983	1,091,078
Gross Unrealized Losses, Less than 12 Months	5,718	7,703
Fair Value, 12 Months or Greater	214,436	190,467
Gross Unrealized Losses, 12 Months or Greater	6,356	6,711
Fair Value, Total	1,133,419	1,281,545
Gross Unrealized Losses, Total	12,074	14,414
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	337,940	1,232,940
Gross Unrealized Losses, Less than 12 Months	2,152	35,406
Fair Value, 12 Months or Greater	98,162	76,797
Gross Unrealized Losses, 12 Months or Greater	7,722	8,987
Fair Value, Total	436,102	1,309,737
Gross Unrealized Losses, Total	9,874	44,393
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	52,233	169,190
Gross Unrealized Losses, Less than 12 Months	640	8,822
Fair Value, 12 Months or Greater	30,842	19,528
Gross Unrealized Losses, 12 Months or Greater	1,850	3,867
Fair Value, Total	83,075	188,718
Gross Unrealized Losses, Total	2,490	12,689
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	1,832,890	3,478,806
Gross Unrealized Losses, Less than 12 Months	11,735	60,461
Fair Value, 12 Months or Greater	626,541	642,780
Gross Unrealized Losses, 12 Months or Greater	21,994	30,945
Fair Value, Total	2,459,431	4,121,586
Gross Unrealized Losses, Total	33,729	91,406
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	46,135	0
Gross Unrealized Losses, Less than 12 Months	8,199	0
Fair Value, 12 Months or Greater	24,845	22,320
Gross Unrealized Losses, 12 Months or Greater	829	3,353
Fair Value, Total	70,980	22,320
Gross Unrealized Losses, Total	9,028	3,353
Equity securities available for sale
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	64,247	18,641
Gross Unrealized Losses, Less than 12 Months	8,978	18,005
Fair Value, 12 Months or Greater	32,961	30,149
Gross Unrealized Losses, 12 Months or Greater	2,491	4,537
Fair Value, Total	97,208	48,790
Gross Unrealized Losses, Total	11,469	22,542
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	18,112	18,641
Gross Unrealized Losses, Less than 12 Months	779	18,005
Fair Value, 12 Months or Greater	8,116	7,829
Gross Unrealized Losses, 12 Months or Greater	1,662	1,184
Fair Value, Total	26,228	26,470
Gross Unrealized Losses, Total	2,441	$ 19,189
Noninvestment Grade Investments at Loss Less than 5 Million [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	89,878
Noninvestment Grade Investments at Loss Less than 5 Million [Member] | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	23,921
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	$ 27,531