Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,081,099		$ 4,188,734
Gross Unrealized Gains	24,523		46,793
Gross Unrealized Losses	(85,450)	[1]	(5,252)	[1]
Fair Value	5,020,172		4,230,275
Percentage of foreign investments held in government and semi-government securities (in hundredths)	84.00%
Percentage of foreign investments held in corporate securities (in hundredths)	12.00%
Percentage Of Foreign Investments Held In Cash equivalents (in hundredths)	4.00%
Percentage of Australian portfolio rated AAA	93.00%
Percentage of Australian portfolio rated AA	7.00%
Maturities, Amortized Cost [Abstract]
Due in one year or less	793,035
Due after one year through five years	1,876,491
Due after five years through ten years	870,872
Due after ten years	480,431
Total debt securities with single maturity date	4,020,829
Total at end of period	5,078,232		4,185,937
Maturities, Fair Value [Abstract]
Due in one year or less	794,081
Due after one year through five years	1,881,820
Due after five years through ten years	846,993
Due after ten years	461,703
Total debt securities with single maturity date	3,984,597
Total at end of period	5,017,288
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	2,924,467		893,029
12 months or greater	47,773		24,298
Total investment portfolio	2,972,240		917,327
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	84,426		4,917
12 months or greater	1,024		335
Total investment portfolio	85,450		5,252
U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	803,637		863,282
Gross Unrealized Gains	1,853		3,040
Gross Unrealized Losses	(18,723)	[1]	(71)	[1]
Fair Value	786,767		866,251
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	466,889		24,094
12 months or greater	0		0
Total investment portfolio	466,889		24,094
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	18,723		71
12 months or greater	0		0
Total investment portfolio	18,723		71
Obligations of U.S. States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	920,006		795,935
Gross Unrealized Gains	7,294		16,965
Gross Unrealized Losses	(14,467)	[1]	(506)	[1]
Fair Value	912,833		812,394
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	450,898		156,111
12 months or greater	1,726		1,006
Total investment portfolio	452,624		157,117
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	14,426		505
12 months or greater	41		1
Total investment portfolio	14,467		506
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,180,842		1,469,844
Gross Unrealized Gains	8,884		13,813
Gross Unrealized Losses	(25,268)	[1]	(2,716)	[1]
Fair Value	2,164,458		1,480,941
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,265,779		280,765
12 months or greater	14,089		3,353
Total investment portfolio	1,279,868		284,118
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	24,940		2,714
12 months or greater	328		2
Total investment portfolio	25,268		2,716
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	395,664		451,352
Gross Unrealized Gains	162		871
Gross Unrealized Losses	(18,261)	[1]	(1,314)	[1]
Fair Value	377,565		450,909
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	395,664
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	377,565
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	348,693		315,000
12 months or greater	24,744		19,939
Total investment portfolio	373,437		334,939
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	17,796		982
12 months or greater	465		332
Total investment portfolio	18,261		1,314
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	260,130		150,232
Gross Unrealized Gains	90		524
Gross Unrealized Losses	(6,413)	[1]	(414)	[1]
Fair Value	253,807		150,342
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	260,130
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	253,807
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	221,265		72,689
12 months or greater	177		0
Total investment portfolio	221,442		72,689
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	6,413		414
12 months or greater	0		0
Total investment portfolio	6,413		414
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	61,336
Gross Unrealized Gains	0
Gross Unrealized Losses	(831)	[1]
Fair Value	60,505
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	61,336
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	60,505
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	60,505
12 months or greater	0
Total investment portfolio	60,505
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	831
12 months or greater	0
Total investment portfolio	831
Debt securities issued by foreign sovereign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	116,344		132,490
Gross Unrealized Gains	5,162		9,784
Gross Unrealized Losses	(967)	[1]	(208)	[1]
Fair Value	120,539		142,066
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	25,461		14,695
12 months or greater	7,037		0
Total investment portfolio	32,498		14,695
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	777		208
12 months or greater	190		0
Total investment portfolio	967		208
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,078,232		4,185,937
Gross Unrealized Gains	24,494		46,654
Gross Unrealized Losses	(85,438)	[1]	(5,252)	[1]
Fair Value	5,017,288		4,227,339
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,867		2,797
Gross Unrealized Gains	29		139
Gross Unrealized Losses	(12)	[1]	0	[1]
Fair Value	2,884		2,936
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	977
12 months or greater	0
Total investment portfolio	977
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	12
12 months or greater	0
Total investment portfolio	12
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	340,273		322,802
Gross Unrealized Gains	1,049		1,657
Gross Unrealized Losses	(508)	[1]	(23)	[1]
Fair Value	340,814		324,436
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	340,273
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	340,814
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	84,000		29,675
12 months or greater	0		0
Total investment portfolio	84,000		29,675
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	508		23
12 months or greater	0		0
Total investment portfolio	$ 508		$ 23

[1] At September 30, 2013 and December 31, 2012, there were no other-than-temporary impairment losses recorded in other comprehensive income.