Investments (Details) (USD $) | 3 Months Ended | 12 Months Ended |
Mar. 31, 2015 | Dec. 31, 2014 |
Securities | 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 |