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Debt (Details) - USD ($) |  |  |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 29, 2018 | Jun. 12, 2017 | Feb. 24, 2015 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jul. 02, 2018 | Jun. 30, 2018 | Jun. 08, 2018 | Feb. 27, 2014
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  | $ 909,000,000 | $ 592,300,000 |  |  |  |  |
Debt Instrument, Collateral Amount |  |  |  | 15,700,000 |  |  |  |  |  |
Loss on Cash Flow Hedge Ineffectiveness |  |  |  | 1,100,000 |  |  |  |  |  |
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months |  |  |  | (300,000) |  |  |  |  |  |
Interest Expense |  |  |  | 43,300,000 | 34,900,000 | $ 44,400,000 |  |  |  |
Interest Paid, Including Capitalized Interest, Operating and Investing Activities |  |  |  | 37,900,000 | 34,700,000 | $ 44,200,000 |  |  |  |
Secured Debt [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  | $ 250,000,000 |
Issuance costs | $ 249,400,000 |  |  |  |  |  |  |  |  |
Notes Payable under Revolving Credit Agreement |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Line of credit, outstanding |  |  |  | 0 |  |  |  |  |  |
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate, percentage |  |  |  |  |  |  |  |  |  | 7.375%
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  |  | 450,000,000 |  |  |  |  |  |
Debt instrument, face amount |  | $ 200,000,000 | $ 250,000,000.0 |  |  |  |  |  |  |
Stated interest rate, percentage |  |  | 4.35% |  |  |  |  |  |  |
Issuance costs |  | $ 200,200,000 | $ 247,300,000 |  |  |  |  |  |  |
Interest rate, effective percentage |  | 4.16% | 4.49% |  |  |  |  |  |  |
Senior Notes [Member] | Senior Notes, 6% Due November 30, 2015 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  | $ 250,000,000 |  |  |  |  |  |  |
Stated interest rate, percentage |  |  | 6.00% |  |  |  |  |  |  |
Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  |  |  |  |  |  | 300,000,000 |
Revolving Credit Facility [Member] | Credit facility and Secured Loan [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Additional Borrowing Capacity, Available |  |  |  |  |  |  |  |  | $ 100,000,000 |
Federal Home Loan Bank of Chicago [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances |  |  |  | 10,000,000 | 0 |  |  | $ 55,000,000 |  |
Federal Home Loan Bank Stock |  |  |  | 800,000 | 400,000 |  |  |  |  |
Federal Home Loan Bank of Dallas [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances |  |  |  | 0 | 0 |  |  | $ 55,000,000 |  |
Federal Home Loan Bank Stock |  |  |  | 3,300,000 | $ 3,300,000 |  |  |  |  |
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Derivative Instruments, Effective Portion Of Accumulated Loss at Discontinuance, Before Tax |  |  |  | 4,500,000 |  |  |  |  |  |
Interest Expense, Other |  |  |  | $ 300,000 |  |  |  |  |  |
Infinity [Member] | Senior Notes [Member] | Senior Note Due on September Nineteen,Twenty Twenty Two [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  | $ 275,000,000 |  |  |
Stated interest rate, percentage |  |  |  |  |  |  | 5.00% |  |  |
Interest rate, effective percentage |  |  |  |  |  |  | 4.36% |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt |  |  |  |  |  |  | $ 282,100,000 |  |  |
Debt Instrument, Unamortized Premium |  |  |  |  |  |  | $ 7,100,000 |  |  |

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- Definition Amount of cash paid for interest, including, but not limited to, capitalized interest and payment to settle zero-coupon bond attributable to accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount; classified as operating and investing activities. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4297-108586 + Details
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