Debt (Details)	1 Months Ended		3 Months Ended		9 Months Ended
	Aug. 31, 2016 USD ($)	Feb. 29, 2016 USD ($)	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) trading_day $ / shares shares	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Loss on debt extinguishment			$ 75,223,000	$ 0	$ 90,531,000	$ 0
Federal Home Loan Bank advances		$ 155,000,000
Federal Home Loan Bank, advances, interest rate (as a percent)		1.91%	1.91%		1.91%
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent)		102.00%
Deferred issuance costs			$ 11,500,000		$ 11,500,000		$ 11,200,000
Convertible senior notes (note 3)			349,073,000		349,073,000		822,301,000
Convertible junior subordinated debentures (note 3)			256,872,000		256,872,000		389,522,000
Long-term debt, par value			1,189,500,000		1,189,500,000		1,223,000,000
Long-term debt, carrying value			1,178,000,000		1,178,000,000		1,211,800,000
Holding company cash and investments			$ 329,000,000		$ 329,000,000
Modified duration of holding company investments (in years)					1 year 7 months 6 days
Interest payments made					$ 31,500,000	$ 32,800,000
Scenario, Previously Reported
Debt Instrument [Line Items]
Long-term debt, carrying value							1,223,000,000
Restatement Adjustment
Debt Instrument [Line Items]
Long-term debt, carrying value							(11,200,000)
Adjustments for New Accounting Pronouncement | Long-term Debt
Debt Instrument [Line Items]
Deferred issuance costs							11,200,000
Adjustments for New Accounting Pronouncement | Other Assets
Debt Instrument [Line Items]
Deferred issuance costs							$ (11,200,000)
Senior Notes | 5.75% Senior Notes
Debt Instrument [Line Items]
Stated interest rate (in hundredths)	5.75%		5.75%		5.75%		5.75%
Aggregate principal amount	$ 425,000,000
Net proceeds from issuance of debt	$ 418,100,000
Redemption price, percentage (in hundredths)	100.00%
Expenses associated with issuance of notes	$ 1,200,000
Ownership threshold for declaration of due and payable (at least) (as a percent)	25.00%
Long-term debt, par value			$ 425,000,000		$ 425,000,000		$ 0
Senior Notes | 5.75% Senior Notes | U.S. Treasury Rate
Debt Instrument [Line Items]
Basis spread on redemption premium (as a percent)	0.50%
Convertible Senior Notes
Debt Instrument [Line Items]
Stated interest rate (in hundredths)			5.00%	5.00%	5.00%	5.00%	5.00%
Extinguishment of debt amount					$ 188,500,000
Purchase price of debt purchased					$ 195,500,000
Reduction in shares for computation of earnings per share (in shares) | shares					14,000,000
Convertible senior notes (note 3)							$ 331,500,000
Long-term debt, par value			$ 145,000,000		$ 145,000,000		333,500,000
Conversion rate (in shares per $1,000 note) | shares					74.4186
Principal amount of notes used in determining conversion rate			$ 1,000		$ 1,000
Initial conversion price (in dollars per share) | $ / shares			$ 13.44		$ 13.44
Interest payments made					$ 6,900,000	$ 8,600,000
Convertible Senior Notes | Scenario, Previously Reported
Debt Instrument [Line Items]
Convertible senior notes (note 3)							333,500,000
Convertible Senior Notes | Restatement Adjustment
Debt Instrument [Line Items]
Convertible senior notes (note 3)							(2,000,000)
Convertible Senior Notes | 2% Convertible Senior Notes
Debt Instrument [Line Items]
Stated interest rate (in hundredths)			2.00%		2.00%
Extinguishment of debt amount			$ 292,400,000
Repayments of debt, aggregate repurchase price			362,100,000
Purchase price of debt purchased			$ 230,700,000
Shares issued for repayment of debt | shares			18,300,000
Reduction in shares for computation of earnings per share (in shares) | shares			42,100,000
9% Debentures
Debt Instrument [Line Items]
Stated interest rate (in hundredths)			9.00%	9.00%	9.00%	9.00%
Extinguishment of debt amount		$ 132,700,000
Purchase price of debt purchased		$ 150,700,000
Reduction in shares for computation of earnings per share (in shares) | shares					9,800,000
Loss on debt extinguishment					$ 8,300,000
Reacquisition of convertible junior subordinated debentures-equity component					$ (6,300,000)
Convertible junior subordinated debentures (note 3)							389,500,000
9% Debentures | Scenario, Previously Reported
Debt Instrument [Line Items]
Convertible junior subordinated debentures (note 3)							389,500,000
9% Debentures | Restatement Adjustment
Debt Instrument [Line Items]
Convertible junior subordinated debentures (note 3)							$ 0
Convertible Junior Subordinated Debentures, interest at 9% per annum, due April 2063
Debt Instrument [Line Items]
Stated interest rate (in hundredths)		9.00%	9.00%	9.00%	9.00%	9.00%	9.00%
Long-term debt, par value			$ 256,900,000		$ 256,900,000		$ 389,500,000
Conversion rate (in shares per $1,000 note) | shares					74.0741
Principal amount of notes used in determining conversion rate			$ 1,000		$ 1,000
Initial conversion price (in dollars per share) | $ / shares			$ 13.50		$ 13.50
Period preceding election to convert					5 days
Interest payments made					$ 15,900,000	$ 17,500,000
2% Notes
Debt Instrument [Line Items]
Stated interest rate (in hundredths)			2.00%	2.00%	2.00%	2.00%	2.00%
Redemption price, percentage (in hundredths)					100.00%
Convertible senior notes (note 3)							$ 490,800,000
Long-term debt, par value			$ 207,600,000		$ 207,600,000		500,000,000
Conversion rate (in shares per $1,000 note) | shares					143.8332
Principal amount of notes used in determining conversion rate			$ 1,000		$ 1,000
Initial conversion price (in dollars per share) | $ / shares			$ 6.95		$ 6.95
Threshold number of trading days (in days) | trading_day					20
Threshold consecutive trading days (in days)					30 days
Percentage of conversion price (in hundredths)			130.00%		130.00%
Convertible debt, conversion price (in dollars per share) | $ / shares			$ 9.03		$ 9.03
Interest payments made					$ 7,000,000	$ 5,000,000
2% Notes | Scenario, Previously Reported
Debt Instrument [Line Items]
Convertible senior notes (note 3)							500,000,000
2% Notes | Restatement Adjustment
Debt Instrument [Line Items]
Convertible senior notes (note 3)							(9,200,000)
FHLB Advance
Debt Instrument [Line Items]
Long-term debt, par value			$ 155,000,000		155,000,000		$ 0
Interest payments made					$ 1,700,000	$ 0
Senior Notes, interest at 5.375% per annum, due November 2015
Debt Instrument [Line Items]
Stated interest rate (in hundredths)			5.375%	5.375%	5.375%	5.375%
Interest payments made					$ 0	$ 1,700,000