Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,509,688		$ 4,188,734
Gross Unrealized Gains	43,714		46,793
Gross Unrealized Losses	(12,712)	[1]	(5,252)	[1]
Fair Value	4,540,690		4,230,275
Percentage of foreign investments held in government and semi-government securities (in hundredths)	87.00%
Percentage of foreign investments held in corporate securities (in hundredths)	11.00%
Percentage Of Foreign Investments Held In Cash equivalents (in hundredths)	2.00%
Percentage of Australian portfolio rated AAA	92.00%
Percentage of Australian portfolio rated AA	8.00%
Maturities, Amortized Cost [Abstract]
Due in one year or less	959,410
Due after one year through five years	1,558,703
Due after five years through ten years	600,924
Due after ten years	372,533
Total debt securities with single maturity date	3,491,570
Total at end of period	4,506,873		4,185,937
Maturities, Fair Value [Abstract]
Due in one year or less	961,273
Due after one year through five years	1,581,603
Due after five years through ten years	608,277
Due after ten years	376,346
Total debt securities with single maturity date	3,527,499
Total at end of period	4,537,762
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,308,101		893,029
12 months or greater	21,509		24,298
Total investment portfolio	1,329,610		917,327
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	12,355		4,917
12 months or greater	357		335
Total investment portfolio	12,712		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	882,336		863,282
Gross Unrealized Gains	2,967		3,040
Gross Unrealized Losses	(568)	[1]	(71)	[1]
Fair Value	884,735		866,251
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	107,846		24,094
12 months or greater	0		0
Total investment portfolio	107,846		24,094
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	568		71
12 months or greater	0		0
Total investment portfolio	568		71
Obligations of U.S. States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	783,756		795,935
Gross Unrealized Gains	16,069		16,965
Gross Unrealized Losses	(383)	[1]	(506)	[1]
Fair Value	799,442		812,394
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	102,228		156,111
12 months or greater	56		1,006
Total investment portfolio	102,284		157,117
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	383		505
12 months or greater	0		1
Total investment portfolio	383		506
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,693,146		1,469,844
Gross Unrealized Gains	13,683		13,813
Gross Unrealized Losses	(3,649)	[1]	(2,716)	[1]
Fair Value	1,703,180		1,480,941
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	547,075		280,765
12 months or greater	3,321		3,353
Total investment portfolio	550,396		284,118
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	3,646		2,714
12 months or greater	3		2
Total investment portfolio	3,649		2,716
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	432,977		451,352
Gross Unrealized Gains	923		871
Gross Unrealized Losses	(5,940)	[1]	(1,314)	[1]
Fair Value	427,960		450,909
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	432,977
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	427,960
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	307,541		315,000
12 months or greater	18,132		19,939
Total investment portfolio	325,673		334,939
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	5,586		982
12 months or greater	354		332
Total investment portfolio	5,940		1,314
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	232,936		150,232
Gross Unrealized Gains	407		524
Gross Unrealized Losses	(1,680)	[1]	(414)	[1]
Fair Value	231,663		150,342
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	232,936
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	231,663
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	139,102		72,689
12 months or greater	0		0
Total investment portfolio	139,102		72,689
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	1,680		414
12 months or greater	0		0
Total investment portfolio	1,680		414
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,897
Gross Unrealized Gains	4
Gross Unrealized Losses	0	[1]
Fair Value	19,901
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	19,897
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	19,901
Debt securities issued by foreign sovereign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	132,332		132,490
Gross Unrealized Gains	8,231		9,784
Gross Unrealized Losses	(421)	[1]	(208)	[1]
Fair Value	140,142		142,066
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	22,711		14,695
12 months or greater	0		0
Total investment portfolio	22,711		14,695
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	421		208
12 months or greater	0		0
Total investment portfolio	421		208
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,506,873		4,185,937
Gross Unrealized Gains	43,599		46,654
Gross Unrealized Losses	(12,710)	[1]	(5,252)	[1]
Fair Value	4,537,762		4,227,339
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,815		2,797
Gross Unrealized Gains	115		139
Gross Unrealized Losses	(2)	[1]	0	[1]
Fair Value	2,928		2,936
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	135
12 months or greater	0
Total investment portfolio	135
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	2
12 months or greater	0
Total investment portfolio	2
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	329,493		322,802
Gross Unrealized Gains	1,315		1,657
Gross Unrealized Losses	(69)	[1]	(23)	[1]
Fair Value	330,739		324,436
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	329,493
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	330,739
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	81,463		29,675
12 months or greater	0		0
Total investment portfolio	81,463		29,675
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	69		23
12 months or greater	0		0
Total investment portfolio	$ 69		$ 23

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