Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	$ 5,823,647		$ 7,458,282
Real estate acquired	1,621	[1]	6,220	[1]
Obligations of U.S. States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	295,690		370,341
Amount of total losses included in earnings for the period attributable to the change in unrealized losses on assets still held at period end	0		0
Total realized/unrealized losses [Abstract]
Included in earnings and reported as realized investment gains (losses), net	(7,883)		0
Included in earnings and reported as net impairment losses recognized in earnings	0		0
Included in earnings and reported as losses incurred, net	0		0
Included in other comprehensive income	6,894		4,913
Purchases	0		0
Sales	(180,475)		(79,564)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Balance at end of period	114,226		295,690
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	70,053		129,338
Amount of total losses included in earnings for the period attributable to the change in unrealized losses on assets still held at period end	0		0
Total realized/unrealized losses [Abstract]
Included in earnings and reported as realized investment gains (losses), net	200		(2,880)
Included in earnings and reported as net impairment losses recognized in earnings	(662)		(2,677)
Included in earnings and reported as losses incurred, net	0		0
Included in other comprehensive income	637		5,342
Purchases	0		0
Sales	(10,000)		(59,070)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Balance at end of period	60,228		70,053
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	321		321
Amount of total losses included in earnings for the period attributable to the change in unrealized losses on assets still held at period end	0		0
Total realized/unrealized losses [Abstract]
Included in earnings and reported as realized investment gains (losses), net	0		0
Included in earnings and reported as net impairment losses recognized in earnings	0		0
Included in earnings and reported as losses incurred, net			0
Included in other comprehensive income	0		0
Purchases	0		0
Sales	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Balance at end of period	321		321
Total Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	366,064		500,000
Amount of total losses included in earnings for the period attributable to the change in unrealized losses on assets still held at period end	0		0
Total realized/unrealized losses [Abstract]
Included in earnings and reported as realized investment gains (losses), net	(7,683)		(2,880)
Included in earnings and reported as net impairment losses recognized in earnings	(662)		(2,677)
Included in earnings and reported as losses incurred, net	0		0
Included in other comprehensive income	7,531		10,255
Purchases	0		0
Sales	(190,475)		(138,634)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Balance at end of period	174,775		366,064
Real Estate Acquired [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	6,220		3,830
Amount of total losses included in earnings for the period attributable to the change in unrealized losses on assets still held at period end	0		0
Total realized/unrealized losses [Abstract]
Included in earnings and reported as realized investment gains (losses), net	0		0
Included in earnings and reported as net impairment losses recognized in earnings	0		0
Included in earnings and reported as losses incurred, net	(371)		(1,926)
Included in other comprehensive income	0		0
Purchases	5,279		15,606
Sales	(9,507)		(11,290)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Balance at end of period	1,621		6,220
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	748,894		1,243,529
Real estate acquired	0	[1]	0	[1]
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	4,899,978		5,848,689
Real estate acquired	0	[1]	0	[1]
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	174,775		366,064
Real estate acquired	1,621	[1]	6,220	[1]
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	597,037		1,102,786
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	597,037		1,102,786
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	0		0
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	0		0
Obligations of U.S. States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	2,323,471		3,580,066
Obligations of U.S. States and Political Subdivisions [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	0		0
Obligations of U.S. States and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	2,209,245		3,284,376
Obligations of U.S. States and Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	114,226		295,690
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	2,032,851		2,564,959
Corporate Debt Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	1,455		2,563
Corporate Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	1,971,168		2,492,343
Corporate Debt Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	60,228		70,053
Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	445,417		57,100
Residential Mortgage-Backed Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	0		0
Residential Mortgage-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	445,417		57,100
Residential Mortgage-Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	0		0
Debt Securities Issued by Foreign Sovereign Governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	157,190		150,327
Debt Securities Issued by Foreign Sovereign Governments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	147,976		135,457
Debt Securities Issued by Foreign Sovereign Governments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	9,214		14,870
Debt Securities Issued by Foreign Sovereign Governments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	0		0
Total Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	5,820,900		7,455,238
Total Debt Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	746,468		1,240,806
Total Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	4,899,978		5,848,689
Total Debt Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	174,454		365,743
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	2,747		3,044
Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	2,426		2,723
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	0		0
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	321		321
Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	264,934
Commercial Mortgage-Backed Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	0
Commercial Mortgage-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	264,934
Commercial Mortgage-Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	$ 0

[1] Real estate acquired through claim settlement, which is held for sale, is reported in Other Assets on the consolidated balance sheet.