Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,067,081		$ 4,188,734
Gross Unrealized Gains	22,172		46,793
Gross Unrealized Losses	(90,262)	[1]	(5,252)	[1]
Fair Value	4,998,991		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	937,314
Due after one year through five years	1,745,300
Due after five years through ten years	878,568
Due after ten years	486,879
Total debt securities with single maturity date	4,048,061
Total at end of period	5,064,233		4,185,937
Maturities, Fair Value [Abstract]
Due in one year or less	938,656
Due after one year through five years	1,743,411
Due after five years through ten years	854,727
Due after ten years	473,120
Total debt securities with single maturity date	4,009,914
Total at end of period	4,996,131
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	3,197,802		893,029
12 months or greater	20,338		24,298
Total investment portfolio	3,218,140		917,327
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	89,794		4,917
12 months or greater	468		335
Total investment portfolio	90,262		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	923,585		863,282
Gross Unrealized Gains	1,935		3,040
Gross Unrealized Losses	(15,397)	[1]	(71)	[1]
Fair Value	910,123		866,251
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	484,399		24,094
12 months or greater	0		0
Total investment portfolio	484,399		24,094
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	15,397		71
12 months or greater	0		0
Total investment portfolio	15,397		71
Obligations of U.S. States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	920,974		795,935
Gross Unrealized Gains	8,721		16,965
Gross Unrealized Losses	(13,616)	[1]	(506)	[1]
Fair Value	916,079		812,394
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	456,902		156,111
12 months or greater	0		1,006
Total investment portfolio	456,902		157,117
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	13,616		505
12 months or greater	0		1
Total investment portfolio	13,616		506
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,086,103		1,469,844
Gross Unrealized Gains	5,183		13,813
Gross Unrealized Losses	(29,548)	[1]	(2,716)	[1]
Fair Value	2,061,738		1,480,941
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,450,667		280,765
12 months or greater	3,291		3,353
Total investment portfolio	1,453,958		284,118
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	29,546		2,714
12 months or greater	2		2
Total investment portfolio	29,548		2,716
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	413,083		451,352
Gross Unrealized Gains	171		871
Gross Unrealized Losses	(21,167)	[1]	(1,314)	[1]
Fair Value	392,087		450,909
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	413,083
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	392,087
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	373,524		315,000
12 months or greater	17,047		19,939
Total investment portfolio	390,571		334,939
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	20,701		982
12 months or greater	466		332
Total investment portfolio	21,167		1,314
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	230,935		150,232
Gross Unrealized Gains	40		524
Gross Unrealized Losses	(8,599)	[1]	(414)	[1]
Fair Value	222,376		150,342
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	230,935
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	222,376
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	206,460		72,689
12 months or greater	0		0
Total investment portfolio	206,460		72,689
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	8,599		414
12 months or greater	0		0
Total investment portfolio	8,599		414
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	61,335
Gross Unrealized Gains	0
Gross Unrealized Losses	(109)	[1]
Fair Value	61,226
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	61,335
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	61,226
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	28,266
12 months or greater	0
Total investment portfolio	28,266
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	109
12 months or greater	0
Total investment portfolio	109
Debt securities issued by foreign sovereign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	117,399		132,490
Gross Unrealized Gains	5,516		9,784
Gross Unrealized Losses	(941)	[1]	(208)	[1]
Fair Value	121,974		142,066
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	32,037		14,695
12 months or greater	0		0
Total investment portfolio	32,037		14,695
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	941		208
12 months or greater	0		0
Total investment portfolio	941		208
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,064,233		4,185,937
Gross Unrealized Gains	22,147		46,654
Gross Unrealized Losses	(90,249)	[1]	(5,252)	[1]
Fair Value	4,996,131		4,227,339
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,848		2,797
Gross Unrealized Gains	25		139
Gross Unrealized Losses	(13)	[1]	0	[1]
Fair Value	2,860		2,936
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	988
12 months or greater	0
Total investment portfolio	988
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	13
12 months or greater	0
Total investment portfolio	13
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	310,819		322,802
Gross Unrealized Gains	581		1,657
Gross Unrealized Losses	(872)	[1]	(23)	[1]
Fair Value	310,528		324,436
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	310,819
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	310,528
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	164,559		29,675
12 months or greater	0		0
Total investment portfolio	164,559		29,675
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	872		23
12 months or greater	0		0
Total investment portfolio	$ 872		$ 23

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