Debt (Details)	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 29, 2016 USD ($)	Jun. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) shares	Jun. 30, 2016 USD ($) trading_day $ / shares shares	Jun. 30, 2015 USD ($) shares	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Antidilutive securities (in shares) | shares		0	28,900,000	0	28,900,000
Loss on debt extinguishment		$ 1,868,000	$ 0	$ 15,308,000	$ 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%	1.91%	1.91%	1.91%
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent)	102.00%
Deferred issuance costs reclassified		$ (8,700,000)		$ (8,700,000)		$ (11,200,000)
Convertible senior notes		636,324,000		636,324,000		822,301,000
Convertible junior subordinated debentures		256,872,000		256,872,000		389,522,000
Long-term debt, par value		1,056,900,000		1,056,900,000		1,223,000,000
Long-term debt, carrying value		1,048,200,000		1,048,200,000		1,211,800,000
Holding company cash and investments		217,000,000		217,000,000
Unrealized gain loss on holding company investments		500,000		$ 500,000
Modified duration of holding company investments (in years)				1 year 4 months 24 days
Interest payments made				$ 28,700,000	$ 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 reclassified						(11,200,000)
Adjustments for New Accounting Pronouncement | Other Assets
Debt Instrument [Line Items]
Deferred issuance costs reclassified						11,200,000
Convertible Senior Notes, interest at 5% per annum, due May 2017
Debt Instrument [Line Items]
Extinguishment of debt amount				188,500,000
Purchase price of debt purchased				$ 195,500,000
Antidilutive securities (in shares) | shares				14,000,000
Convertible senior notes						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
Stated interest rate (in hundredths)		5.00%	5.00%	5.00%	5.00%	5.00%
Interest payments made				$ 6,900,000	$ 8,600,000
Convertible Senior Notes, interest at 5% per annum, due May 2017 | Scenario, Previously Reported
Debt Instrument [Line Items]
Convertible senior notes						$ 333,500,000
Convertible Senior Notes, interest at 5% per annum, due May 2017 | Restatement Adjustment
Debt Instrument [Line Items]
Convertible senior notes						(2,000,000)
Convertible Junior Subordinated Debentures, interest at 9% per annum, due April 2063
Debt Instrument [Line Items]
Extinguishment of debt amount	$ 132,700,000
Purchase price of debt purchased	$ 150,700,000
Antidilutive securities (in shares) | shares				9,800,000
Loss on debt extinguishment				$ 8,300,000
Reacquisition of convertible junior subordinated debentures-equity component				$ (9,800,000)
Convertible junior subordinated debentures						389,500,000
Stated interest rate (in hundredths)		9.00%	9.00%	9.00%	9.00%
Convertible Junior Subordinated Debentures, interest at 9% per annum, due April 2063 | Scenario, Previously Reported
Debt Instrument [Line Items]
Convertible junior subordinated debentures						389,500,000
Convertible Junior Subordinated Debentures, interest at 9% per annum, due April 2063 | Restatement Adjustment
Debt Instrument [Line Items]
Convertible junior subordinated debentures						0
Convertible Senior Notes, interest at 2% per annum, due April 2020
Debt Instrument [Line Items]
Convertible senior notes						490,800,000
Long-term debt, par value		$ 500,000,000		$ 500,000,000		$ 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
Stated interest rate (in hundredths)		2.00%	2.00%	2.00%	2.00%	2.00%
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
Redemption price, percentage (in hundredths)				100.00%
Interest payments made				$ 5,000,000	$ 5,000,000
Convertible Senior Notes, interest at 2% per annum, due April 2020 | Scenario, Previously Reported
Debt Instrument [Line Items]
Convertible senior notes						$ 500,000,000
Convertible Senior Notes, interest at 2% per annum, due April 2020 | Restatement Adjustment
Debt Instrument [Line Items]
Convertible senior notes						(9,200,000)
FHLB Advance, interest at 1.91% per annum, due February 2023
Debt Instrument [Line Items]
Long-term debt, par value		$ 155,000,000		155,000,000		0
Interest payments made				900,000	$ 0
Convertible Junior Subordinated Debentures, interest at 9% per annum, due April 2063
Debt Instrument [Line Items]
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
Stated interest rate (in hundredths)		9.00%	9.00%	9.00%	9.00%	9.00%
Period preceding election to convert				5 days
Interest payments made				$ 15,900,000	$ 17,500,000
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