Investments (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015 Securities		Dec. 31, 2014 Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,570,894,000		$ 4,605,517,000
Gross Unrealized Gains	45,613,000		37,649,000
Gross Unrealized Losses	(18,744,000)	[1]	(30,497,000)	[1]
Fair Value	4,597,763,000		4,612,669,000
Percentage of foreign investments held in government and semi-government securities (in hundredths)	86.00%
Percentage of foreign investments held in corporate securities (in hundredths)	10.00%
Percentage of foreign investments held in cash equivalents (in hundredths)	4.00%
Percentage of Australian portfolio rated AAA (in hundredths)	85.00%
Percentage of Australian portfolio rated AA (in hundredths)	15.00%
Gross unrealized (gains) losses	18,700,000		30,500,000
Maturities, Amortized Cost [Abstract]
Due in one year or less	301,142,000
Due after one year through five years	1,806,892,000
Due after five years through ten years	1,127,078,000
Due after ten years	442,523,000
Total debt securities with single maturity date	3,677,635,000
Total at end of period	4,567,873,000		4,602,514,000
Maturities, Fair Value [Abstract]
Due in one year or less	301,873,000
Due after one year through five years	1,823,593,000
Due after five years through ten years	1,133,449,000
Due after ten years	451,648,000
Total debt securities with single maturity date	3,710,563,000
Total at end of period	4,594,663,000		4,609,614,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,020,671,000		1,209,256,000
12 months or greater	564,303,000		1,035,884,000
Total investment portfolio	1,584,974,000		2,245,140,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	5,708,000		7,388,000
12 months or greater	13,036,000		23,109,000
Total investment portfolio	18,744,000		30,497,000
Number of securities in unrealized loss position	278		423
U.S. Treasury securities and obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	312,233,000		349,153,000
Gross Unrealized Gains	2,629,000		2,752,000
Gross Unrealized Losses	(2,057,000)	[1]	(5,130,000)	[1]
Fair Value	312,805,000		346,775,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	93,555,000		58,166,000
12 months or greater	26,435,000		232,351,000
Total investment portfolio	119,990,000		290,517,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	362,000		138,000
12 months or greater	1,695,000		4,992,000
Total investment portfolio	2,057,000		5,130,000
Obligations of U.S. states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	937,548,000		844,942,000
Gross Unrealized Gains	16,621,000		12,961,000
Gross Unrealized Losses	(1,466,000)	[1]	(2,761,000)	[1]
Fair Value	952,703,000		855,142,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	174,785,000		166,408,000
12 months or greater	50,757,000		114,465,000
Total investment portfolio	225,542,000		280,873,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	695,000		1,066,000
12 months or greater	771,000		1,695,000
Total investment portfolio	1,466,000		2,761,000
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,394,862,000		2,418,991,000
Gross Unrealized Gains	18,939,000		16,325,000
Gross Unrealized Losses	(5,785,000)	[1]	(10,035,000)	[1]
Fair Value	2,408,016,000		2,425,281,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	621,952,000		816,555,000
12 months or greater	113,002,000		243,208,000
Total investment portfolio	734,954,000		1,059,763,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	4,169,000		5,259,000
12 months or greater	1,616,000		4,776,000
Total investment portfolio	5,785,000		10,035,000
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	253,185,000		286,260,000
Gross Unrealized Gains	769,000		535,000
Gross Unrealized Losses	(35,000)	[1]	(140,000)	[1]
Fair Value	253,919,000		286,655,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	253,185,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	253,919,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	34,294,000		54,491,000
12 months or greater	11,930,000		11,895,000
Total investment portfolio	46,224,000		66,386,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	10,000		80,000
12 months or greater	26,000		60,000
Total investment portfolio	36,000		140,000
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	315,778,000		329,983,000
Gross Unrealized Gains	719,000		254,000
Gross Unrealized Losses	(7,466,000)	[1]	(9,000,000)	[1]
Fair Value	309,031,000		321,237,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	315,778,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	309,031,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	34,574,000		24,168,000
12 months or greater	229,563,000		263,002,000
Total investment portfolio	264,137,000		287,170,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	168,000		34,000
12 months or greater	7,298,000		8,966,000
Total investment portfolio	7,466,000		9,000,000
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	259,934,000		276,215,000
Gross Unrealized Gains	1,804,000		1,221,000
Gross Unrealized Losses	(1,132,000)	[1]	(2,158,000)	[1]
Fair Value	260,606,000		275,278,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	259,934,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	260,606,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	61,443,000		89,301,000
12 months or greater	71,895,000		110,652,000
Total investment portfolio	133,338,000		199,953,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	304,000		810,000
12 months or greater	828,000		1,348,000
Total investment portfolio	1,132,000		2,158,000
Collateralized loan obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	61,341,000		61,340,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(797,000)	[1]	(1,264,000)	[1]
Fair Value	60,544,000		60,076,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	61,341,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	60,544,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	0		0
12 months or greater	60,544,000		60,076,000
Total investment portfolio	60,544,000		60,076,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	0		0
12 months or greater	797,000		1,264,000
Total investment portfolio	797,000		1,264,000
Debt securities issued by foreign sovereign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,992,000		35,630,000
Gross Unrealized Gains	4,047,000		3,540,000
Gross Unrealized Losses	0	[1]	0	[1]
Fair Value	37,039,000		39,170,000
Total debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,567,873,000		4,602,514,000
Gross Unrealized Gains	45,528,000		37,588,000
Gross Unrealized Losses	(18,738,000)	[1]	(30,488,000)	[1]
Fair Value	4,594,663,000		4,609,614,000
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,021,000		3,003,000
Gross Unrealized Gains	85,000		61,000
Gross Unrealized Losses	(6,000)	[1]	(9,000)	[1]
Fair Value	3,100,000		3,055,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	68,000		167,000
12 months or greater	177,000		235,000
Total investment portfolio	245,000		402,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	0		1,000
12 months or greater	5,000		8,000
Total investment portfolio	$ 5,000		$ 9,000

[1] At March 31, 2015 and December 31, 2014, there were no other-than-temporary impairment losses recorded in other comprehensive income