Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified																					3 Months Ended		9 Months Ended										3 Months Ended		9 Months Ended																																						3 Months Ended		9 Months Ended														3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2011		Dec. 31, 2010		Sep. 30, 2011 U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]	Dec. 31, 2010 U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]	Sep. 30, 2011 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]	Dec. 31, 2010 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]	Sep. 30, 2011 U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2010 U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2011 U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2010 U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2011 Obligations of U.S. States and Political Subdivisions [Member]	Dec. 31, 2010 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2011 Obligations of U.S. States and Political Subdivisions [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2010 Obligations of U.S. States and Political Subdivisions [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2011 Obligations of U.S. States and Political Subdivisions [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2010 Obligations of U.S. States and Political Subdivisions [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2011 Obligations of U.S. States and Political Subdivisions [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2010 Obligations of U.S. States and Political Subdivisions [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2011 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2010 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2011 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2010 Obligations of U.S. States and Political Subdivisions [Member]	Sep. 30, 2011 Corporate Debt Securities [Member]	Dec. 31, 2010 Corporate Debt Securities [Member]	Sep. 30, 2011 Corporate Debt Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2010 Corporate Debt Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2011 Corporate Debt Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2010 Corporate Debt Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2011 Corporate Debt Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2010 Corporate Debt Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2011 Corporate Debt Securities [Member]	Sep. 30, 2010 Corporate Debt Securities [Member]	Sep. 30, 2011 Corporate Debt Securities [Member]	Sep. 30, 2010 Corporate Debt Securities [Member]	Sep. 30, 2011 Commercial Mortgage-Backed Securities [Member]	Sep. 30, 2011 Commercial Mortgage-Backed Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2011 Commercial Mortgage-Backed Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2011 Commercial Mortgage-Backed Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2011 Residential Mortgage-Backed Securities [Member]	Dec. 31, 2010 Residential Mortgage-Backed Securities [Member]	Sep. 30, 2011 Residential Mortgage-Backed Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2010 Residential Mortgage-Backed Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2011 Residential Mortgage-Backed Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2010 Residential Mortgage-Backed Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2011 Residential Mortgage-Backed Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2010 Residential Mortgage-Backed Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2011 Debt Securities Issued by Foreign Sovereign Governments [Member]	Dec. 31, 2010 Debt Securities Issued by Foreign Sovereign Governments [Member]	Sep. 30, 2011 Debt Securities Issued by Foreign Sovereign Governments [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2010 Debt Securities Issued by Foreign Sovereign Governments [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2011 Debt Securities Issued by Foreign Sovereign Governments [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2010 Debt Securities Issued by Foreign Sovereign Governments [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2011 Debt Securities Issued by Foreign Sovereign Governments [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2010 Debt Securities Issued by Foreign Sovereign Governments [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2011 Total Debt Securities [Member]	Dec. 31, 2010 Total Debt Securities [Member]	Sep. 30, 2011 Total Debt Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2010 Total Debt Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2011 Total Debt Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2010 Total Debt Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2011 Total Debt Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2010 Total Debt Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2011 Equity Securities [Member]	Dec. 31, 2010 Equity Securities [Member]	Sep. 30, 2011 Equity Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2010 Equity Securities [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Sep. 30, 2011 Equity Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2010 Equity Securities [Member] Significant Other Observable Inputs (Level 2) [Member]	Sep. 30, 2011 Equity Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2010 Equity Securities [Member] Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2011 Equity Securities [Member]	Sep. 30, 2010 Equity Securities [Member]	Sep. 30, 2011 Equity Securities [Member]	Sep. 30, 2010 Equity Securities [Member]	Sep. 30, 2011 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]		Dec. 31, 2010 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]		Sep. 30, 2011 Significant Other Observable Inputs (Level 2) [Member]		Dec. 31, 2010 Significant Other Observable Inputs (Level 2) [Member]		Sep. 30, 2011 Significant Unobservable Inputs (Level 3) [Member]		Dec. 31, 2010 Significant Unobservable Inputs (Level 3) [Member]		Sep. 30, 2011 Total Investments [Member]	Sep. 30, 2010 Total Investments [Member]	Sep. 30, 2011 Total Investments [Member]	Sep. 30, 2010 Total Investments [Member]	Sep. 30, 2011 Real Estate Acquired [Member]	Sep. 30, 2010 Real Estate Acquired [Member]	Sep. 30, 2011 Real Estate Acquired [Member]	Sep. 30, 2010 Real Estate Acquired [Member]
Fair value measurements [Abstract]
Discount rates used for the DCF model, minimum (in hundredths)	2.24%
Discount rate used for the DCF model, maximum (in hundredths)	4.24%
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	$ 6,458,220		$ 7,458,282		$ 776,254	$ 1,102,786	$ 776,254	$ 1,102,786	$ 0	$ 0	$ 0	$ 0	$ 2,999,632	$ 3,580,066	$ 0	$ 0	$ 2,778,425	$ 3,284,376	$ 221,207	$ 295,690					$ 2,293,106	$ 2,564,959	$ 2,476	$ 2,563	$ 2,220,340	$ 2,492,343	$ 70,290	$ 70,053					$ 195,033	$ 0	$ 195,033	$ 0	$ 49,835	$ 57,100	$ 0	$ 0	$ 49,835	$ 57,100	$ 0	$ 0	$ 141,661	$ 150,327	$ 132,382	$ 135,457	$ 9,279	$ 14,870	$ 0	$ 0	$ 6,455,521	$ 7,455,238	$ 911,112	$ 1,240,806	$ 5,252,912	$ 5,848,689	$ 291,497	$ 365,743	$ 2,699	$ 3,044	$ 2,378	$ 2,723	$ 0	$ 0	$ 321	$ 321					$ 913,490		$ 1,243,529		$ 5,252,912		$ 5,848,689		$ 291,818		$ 366,064
Real estate acquired	2,324	[1]	6,220	[1]																																																																									0	[1]	0	[1]	0	[1]	0	[1]	2,324	[1]	6,220	[1]
Reconciliation of assets measured at fair value using significant unobservable inputs (Level 3) [Roll Forward]
Balance at beginning of period																					223,402	321,050	295,690	370,341									70,039	94,564	70,053	129,338																																					321	321	321	321													293,762	415,935	366,064	500,000	2,828	5,671	6,220	3,830
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as realized investment losses, net																					0												(200)	(1,057)	(200)	(2,455)																																																					(200)	(1,057)	(200)	(2,455)	(85)		0
Included in earnings and reported as losses incurred, net																																																																																														(701)	(180)	(1,635)
Included in other comprehensive income																					342	3,504	(845)	3,547									451	2,528	437	2,854																																					0	0	0	0													793	6,032	(408)	6,401	0	0	0	0
Purchases																					0		0										0		0																																						0		0														0		0		1,148		3,944
Purchases, issuances and settlements																						(12,858)		(62,192)										(15,793)		(49,495)																																						0		0														(28,651)		(111,687)		1,893		4,668
Sales																					(2,537)	0	(73,638)	0									0	0	0	0																																					0	0	0	0													(2,537)	0	(73,638)	0	(1,567)	0	(7,660)	0
Transfers in and/or out of Level 3																					0	0	0	0									0	0	0	0																																					0	0	0	0													0	0	0	0	0	0	0	0
Balance at end of period																					221,207	311,696	221,207	311,696									70,290	80,242	70,290	80,242																																					321	321	321	321													291,818	392,259	291,818	392,259	2,324	6,863	2,324	6,863
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	$ 0	$ 0									$ 0	$ 0	$ 0	$ 0																																					$ 0	$ 0	$ 0	$ 0													$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0

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