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Debt (Details) | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Aug. 31, 2016 USD ($) | Feb. 29, 2016 USD ($) | Dec. 31, 2016 USD ($) Period $ / shares shares | Sep. 30, 2016 USD ($) shares | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) Period $ / shares shares | Dec. 31, 2015 USD ($) shares | Dec. 31, 2014 USD ($) | Feb. 26, 2016 USD ($) | Nov. 02, 2015
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred issuance costs |  |  | $ 10,800,000 |  |  |  | $ 11,200,000 |  |  |  | $ 10,800,000 | $ 11,200,000 |  |  |
Repayments of Convertible Debt |  |  |  |  |  |  |  |  |  |  | $ 363,778,000 | 11,152,000 | $ 0 |  |
FHLBC advances |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 155,000,000 |
FHLBC fixed interest rate |  | 1.91% |  |  |  |  |  |  |  |  |  |  |  |  |
Collateral, principal balance of FHLB (as a percent) |  | 102.00% |  |  |  |  |  |  |  |  | 102.00% |  |  |  |
Long-term debt, gross |  |  | 1,189,500,000 |  |  |  | 1,223,000,000 |  |  |  | $ 1,189,500,000 | 1,223,000,000 |  |  |
Repayment of long-term debt |  |  |  |  |  |  |  |  |  |  | 0 | 61,953,000 | 20,772,000 |  |
Loss on debt extinguishment |  |  | $ 0 | $ 75,223,000 | $ 1,868,000 | $ 13,440,000 | 507,000 | $ 0 | $ 0 | $ 0 | $ 90,531,000 | 507,000 | $ 837,000 |  |
Senior Notes | 5.75% Senior Notes due 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate (in hundredths) | 5.75% |  | 5.75% |  |  |  |  |  |  |  | 5.75% |  |  |  |
Outstanding principal amount | $ 425,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of debt | $ 418,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Redemption price, percentage (in hundredths) |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |
Other expenses associated with the issuance of notes |  |  | $ 1,200,000 |  |  |  |  |  |  |  | $ 1,200,000 |  |  |  |
Ownership percentage threshold for declaration of due and payable |  |  |  |  |  |  |  |  |  |  | 25.00% |  |  |  |
Long-term debt, gross |  |  | $ 425,000,000 |  |  |  | 0 |  |  |  | $ 425,000,000 | 0 |  |  |
Senior Notes | 5.75% Senior Notes due 2023 | U.S. Treasury Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on redemption premium (as a percent) | 0.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes | 5.375% Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate (in hundredths) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.375%
Convertible Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate (in hundredths) |  |  | 5.00% |  |  |  |  |  |  |  | 5.00% |  |  |  |
Repayments of Convertible Debt |  |  |  |  |  |  |  |  |  |  | $ 195,500,000 | 12,000,000 |  |  |
Long-term debt, gross |  |  | $ 145,000,000 |  |  |  | 333,500,000 |  |  |  | 145,000,000 | 333,500,000 |  |  |
Extinguishment of debt, amount |  |  |  |  |  |  |  |  |  |  | $ 188,500,000 | $ 11,500,000 |  |  |
Reduction in shares for computation of earnings per share | shares |  |  |  |  |  |  |  |  |  |  | 14,000,000 | 900,000 |  |  |
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 |  |  |  |
Convertible debt, number of equity instruments reserved for conversion | shares |  |  | 10,800,000 |  |  |  |  |  |  |  | 10,800,000 |  |  |  |
Minimum percentage of consolidated shareholders' equity that determines designated subsidiary status |  |  | 15.00% |  |  |  |  |  |  |  | 15.00% |  |  |  |
Convertible Debt | 2% Convertible Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate (in hundredths) |  |  | 2.00% |  |  |  |  |  |  |  | 2.00% |  |  |  |
Repayments of Convertible Debt |  |  |  | 230,700,000 |  |  |  |  |  |  |  |  |  |  |
Extinguishment of debt, amount |  |  |  | $ 292,400,000 |  |  |  |  |  |  |  |  |  |  |
Reduction in shares for computation of earnings per share | shares |  |  |  | 42,100,000 |  |  |  |  |  |  |  |  |  |  |
Repayments of long-term debt |  |  |  | $ 362,100,000 |  |  |  |  |  |  |  |  |  |  |
Repayments of debt (in shares) | shares |  |  |  | 18,300,000 |  |  |  |  |  |  |  |  |  |  |
2% Convertible Senior Notes due 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate (in hundredths) |  |  | 2.00% |  |  |  |  |  |  |  | 2.00% |  |  |  |
Redemption price, percentage (in hundredths) |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |
Long-term debt, gross |  |  | $ 207,600,000 |  |  |  | 500,000,000 |  |  |  | $ 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 |  |  |  |
Convertible debt, number of equity instruments reserved for conversion | shares |  |  | 29,900,000 |  |  |  |  |  |  |  | 29,900,000 |  |  |  |
Minimum number of trading days |  |  |  |  |  |  |  |  |  |  | 20 days |  |  |  |
Maximum number of trading days |  |  |  |  |  |  |  |  |  |  | 30 days |  |  |  |
Percentage of conversion price (in hundredths) |  |  | 130.00% |  |  |  |  |  |  |  | 130.00% |  |  |  |
130% of conversion price (in dollars per share) | $ / shares |  |  | $ 9.04 |  |  |  |  |  |  |  | $ 9.04 |  |  |  |
Convertible Junior Subordinated Debentures Due 2063 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate (in hundredths) |  |  | 9.00% |  |  |  |  |  |  |  | 9.00% |  |  |  |
Redemption price, percentage (in hundredths) |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |
Long-term debt, gross |  |  | $ 256,900,000 |  |  |  | 389,500,000 |  |  |  | $ 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 |  |  |  |
Convertible debt, number of equity instruments reserved for conversion | shares |  |  | 19,000,000 |  |  |  |  |  |  |  | 19,000,000 |  |  |  |
Minimum number of trading days |  |  |  |  |  |  |  |  |  |  | 20 days |  |  |  |
Maximum number of trading days |  |  |  |  |  |  |  |  |  |  | 30 days |  |  |  |
Percentage of conversion price (in hundredths) |  |  | 130.00% |  |  |  |  |  |  |  | 130.00% |  |  |  |
130% of conversion price (in dollars per share) | $ / shares |  |  | $ 17.55 |  |  |  |  |  |  |  | $ 17.55 |  |  |  |
Period preceding election to convert |  |  |  |  |  |  |  |  |  |  | 5 days |  |  |  |
Minimum number of consecutive interest periods for which interest payments may be deferred | Period |  |  | 1 |  |  |  |  |  |  |  | 1 |  |  |  |
Maximum period for which interest payments may be deferred without giving rise to an event of default |  |  |  |  |  |  |  |  |  |  | 10 years |  |  |  |
Period in which reasonable commercial efforts must begin, maximum |  |  |  |  |  |  |  |  |  |  | 1 day |  |  |  |
Anniversary payment release of the start of the interest deferral to the Alternative Payment Mechanism in lieu of the final maturity of the debentures |  |  |  |  |  |  |  |  |  |  | 10 years |  |  |  |
Net proceeds cap (in hundredths) |  |  | 2.00% |  |  |  |  |  |  |  | 2.00% |  |  |  |
Maximum number of shares of common stock issuable under the Alternative Payment Mechanism (in shares) | shares |  |  | 10,000,000 |  |  |  |  |  |  |  | 10,000,000 |  |  |  |
Maximum percentage of aggregate principal amount of the debentures (in hundredths) |  |  | 25.00% |  |  |  |  |  |  |  | 25.00% |  |  |  |
Convertible Junior Debentures at 9% per annum, Due 2063 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate (in hundredths) |  |  | 9.00% |  |  |  |  |  |  |  | 9.00% |  |  |  |
Extinguishment of debt, amount |  | $ 132,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Repayment of long-term debt |  | $ 150,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in shares for computation of earnings per share | shares |  |  |  |  |  |  |  |  |  |  | 9,800,000 |  |  |  |
Loss on debt extinguishment |  |  |  |  |  |  |  |  |  |  | $ 8,300,000 |  |  |  |
Reacquisition of convertible junior subordinated debentures-equity component |  |  |  |  |  |  |  |  |  |  | $ 6,300,000 |  |  |  |
Accounting Standards Update 2015-03 | Long-term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred issuance costs |  |  |  |  |  |  | (11,200,000) |  |  |  |  | (11,200,000) |  |  |
Accounting Standards Update 2015-03 | Other Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred issuance costs |  |  |  |  |  |  | $ (11,200,000) |  |  |  |  | $ (11,200,000) |  |  |

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