Summary of significant accounting policies (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Summary of significant accounting policies [Abstract]
Discount rates used for the DCF model, minimum (in hundredths)																	2.30%
Discount rate used for the DCF model, maximum (in hundredths)																	4.30%
Home office and equipment [Abstract]
Accumulated depreciation of home office and equipment	$ 65,200,000								$ 62,900,000								$ 65,200,000		$ 62,900,000		$ 60,100,000
Depreciation expense of home office and equipment																	2,300,000		2,900,000		4,300,000
Loss reserves [Abstract]
Minimum period for which an insured loan is past due in order for it to be considered in default (in days)																	45D
Income taxes [Abstract]
Net premiums threshold for early withdrawals (in hundredths)	35.00%																35.00%
Share-Based Compensation [Abstract]
Minimum vesting period for share-based compensation awards (in years)																	1Y
Maximum vesting period for share-based compensation awards (in years)																	5Y
Earnings per share [Abstract]
Antidilutive securities excluded from weighted average number of shares (in shares)																	55,600,000		47,400,000		37,600,000
Basic (loss) earnings per share [Abstract]
Average common shares outstanding (in shares)																	201,019,000		176,406,000		124,209,000
Net loss	(135,294,000)		(165,205,000)		(151,732,000)		(33,661,000)		(186,667,000)	[1]	(51,528,000)		24,551,000		(150,091,000)		(485,892,000)		(363,735,000)		(1,322,277,000)
Basic (loss) earnings per share (in dollars per share)	$ (0.67)	[2]	$ (0.82)	[2]	$ (0.75)	[2]	$ (0.17)	[2]	$ (0.93)	[1],[2]	$ (0.26)	[2]	$ 0.14	[2]	$ (1.20)	[2]	$ (2.42)	[2]	$ (2.06)	[2]	$ (10.65)
Diluted (loss) earnings per share [Abstract]
Weighted-average shares - Basic (in shares)																	201,019,000		176,406,000		124,209,000
Common stock equivalents (in shares)																	0		0		0
Weighted-average shares - Diluted (in shares)																	201,019,000		176,406,000		124,209,000
Net loss	(135,294,000)		(165,205,000)		(151,732,000)		(33,661,000)		(186,667,000)	[1]	(51,528,000)		24,551,000		(150,091,000)		(485,892,000)		(363,735,000)		(1,322,277,000)
Diluted (loss) earnings per share (in dollars per share)	$ (0.67)	[2]	$ (0.82)	[2]	$ (0.75)	[2]	$ (0.17)	[2]	$ (0.93)	[1],[2]	$ (0.26)	[2]	$ 0.13	[2]	$ (1.20)	[2]	$ (2.42)	[2]	$ (2.06)	[2]	$ (10.65)
Total other comprehensive income (loss) [Abstract]
Net loss	(135,294,000)		(165,205,000)		(151,732,000)		(33,661,000)		(186,667,000)	[1]	(51,528,000)		24,551,000		(150,091,000)		(485,892,000)		(363,735,000)		(1,322,277,000)
Other comprehensive (loss) income																	7,988,000		(52,019,000)		180,944,000
Comprehensive loss																	(477,904,000)		(415,754,000)		(1,141,333,000)
Other comprehensive income (loss) (net of tax) [Abstract]
Change in unrealized gains and losses on investments																	21,057,000		(69,074,000)		154,358,000
Noncredit component of impairment losses, net (note 6)																	0		0		(1,764,000)
Amortization related to benefit plans																	(12,862,000)		6,390,000		10,704,000
Unrealized foreign currency translation adjustment																	(207,000)		10,665,000		17,646,000
Other comprehensive (loss) income																	7,988,000		(52,019,000)		180,944,000
Tax expense (benefit) on other comprehensive income																	3,600,000		5,900,000		98,100,000
Accumulated other comprehensive income (loss) [Abstract]
Unrealized gains (losses) on investments	53,561,000								32,503,000								53,561,000		32,503,000
Defined benefit plans	(43,642,000)								(30,780,000)								(43,642,000)		(30,780,000)
Foreign currency translation adjustment	20,205,000								20,413,000								20,205,000		20,413,000
Total accumulated other comprehensive income	$ 30,124,000								$ 22,136,000								$ 30,124,000		$ 22,136,000
Participating Securities with Non-forfeitable Rights to Dividends [Member]
Earnings per share [Abstract]
Antidilutive securities excluded from weighted average number of shares (in shares)																	1,100,000		1,800,000		1,900,000
Home Office [Member]
Home office and equipment [Abstract]
Estimated useful life (in years)																	45
Equipment [Member]
Home office and equipment [Abstract]
Estimated useful life (in years)																	5
Data Processing Hardware [Member]
Home office and equipment [Abstract]
Estimated useful life (in years)																	3

[1] In prior periods, the liability associated with premium to be returned on claim payments is included in loss reserves and changes to this estimate affect losses incurred. See Note 3 - Summary of significant accounting policies - Revenue recognition.

[2] Due to the use of weighted average shares outstanding when calculating earnings per share, the sum of the quarterly per share data may not equal the per share data for the year.