Income taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2007
Net deferred tax assets and liabilities [Abstract]
Total deferred tax assets	$ 683,645,000	$ 651,568,000
Total deferred tax liabilities	(86,490,000)	(249,989,000)
Net deferred tax asset before valuation allowance	597,155,000	401,579,000
Valuation allowance	(608,761,000)	(410,333,000)	238,500,000
Net deferred tax liability	(11,606,000)	(8,754,000)
Components of deferred tax liability [Abstract]
Convertible debentures	(15,785,000)	(25,864,000)
Net operating loss	506,614,000	432,827,000
Loss reserves	60,478,000	85,425,000
Unrealized (appreciation) depreciation in investments	(42,009,000)	(31,379,000)
Mortgage investments	18,944,000	17,934,000
Deferred compensation	17,447,000	19,080,000
Investments in joint ventures	(3,018,000)	(165,598,000)
Premium deficiency reserves	47,186,000	62,638,000
Loss due to other than temporary impairments	11,068,000	14,160,000
Other, net	(3,770,000)	(7,644,000)
Net deferred tax asset before valuation allowance	597,155,000	401,579,000
Valuation allowance	(608,761,000)	(410,333,000)	238,500,000
Net deferred tax liability	(11,606,000)	(8,754,000)
Income tax valuation allowance [Abstract]
Components of other comprehensive income (loss) that impact changes to valuation allowance	2,300,000	63,500,000	102,300,000
Increase (decrease) in deferred tax valuation allowance, included in other comprehensive income, due to change in deferred tax liability related to unrealized gains/losses on investments	0	22,200,000	0
Tax provision (benefit) [Abstract]
Benefit from income taxes	(196,835,000)	(145,334,000)	(681,266,000)
Change in valuation allowance	198,428,000	149,669,000	238,490,000
Tax provision (benefit)	1,593,000	4,335,000	(442,776,000)
Income tax benefit, operating loss carryback, change in legislation		282,000,000
Net operating loss carryforwards, regular tax basis	1,448,000,000
Net operating loss carryforwards for computing the alternative minimum tax	582,000,000
Components of provisions for (benefit from) income taxes [Abstract]
Current	598,000	1,618,000	(621,170,000)
Deferred	(945,000)	(19,000)	175,194,000
Other	1,940,000	2,736,000	3,200,000
Provision for (benefit from) income taxes	1,593,000	4,335,000	(442,776,000)
Federal income tax received	0	289,100,000	437,500,000
Reconciliation of effective income tax rate [Abstract]
Federal statutory income tax benefit rate (in hundredths)	(35.00%)	(35.00%)	(35.00%)
Valuation allowance (in hundredths)	41.00%	41.60%	13.50%
Tax exempt municipal bond interest (in hundredths)	(5.40%)	(10.50%)	(3.60%)
Other, net (in hundredths)	(0.30%)	5.10%	0.00%
Effective income tax (benefit) rate (in hundredths)	0.30%	1.20%	(25.10%)
Information regarding income tax examinations [Abstract]
Amount of IRS assessment for unpaid taxes and penalties related to REMIC issue				190,700,000
Amount of payment made related to the IRS assessment on the REMIC issue				65,200,000
Unrecognized tax benefits [Roll Forward]
Balance at beginning of year	109,282,000	91,117,000	87,965,000
Additions based on tax positions related to the current year	0	0	258,000
Additions for tax positions of prior years	798,000	18,165,000	2,894,000
Reductions for tax positions of prior years	0	0	0
Settlements	0	0	0
Balance at end of year	110,080,000	109,282,000	91,117,000
Unrecognized tax benefits [Abstract]
Total amount of unrecognized tax benefits that would affect effective tax rate	97,500,000
Unrecognized tax benefits, interest expense	800,000
Unrecognized tax benefits, accrued interest	26,700,000	25,900,000
Expected reduction in unrecognized tax benefits during current year	104,000,000
Expected cash outflows related to income taxes for current year	$ 23,000,000