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Debt (Details) - USD ($) |  |  | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jul. 02, 2018 | Jun. 29, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  | $ 1,123,700,000 |  | $ 1,123,700,000 |  |  |  | $ 592,300,000
Debt Instrument, Collateral Amount |  |  | 15,900,000 |  | 15,900,000 |  |  |  |
Interest Expense |  |  | 13,400,000 | $ 7,800,000 | 29,300,000 | $ 26,900,000 |  |  |
Interest Paid, Including Capitalized Interest, Operating and Investing Activities |  |  | 19,700,000 | $ 12,700,000 | 35,000,000 | $ 31,800,000 |  |  |
Secured Debt [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | 250,000,000 |  | 250,000,000 |  |  |  |
Long-term Debt, Gross |  |  | 249,400,000 |  | 249,400,000 |  |  |  | 0
Proceeds from Debt, Net of Issuance Costs |  | $ 249,400,000 |  |  |  |  |  |  |
Senior Notes [Member] | Senior Note Due on September Nineteen,Twenty Twenty Two [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  | 281,700,000 |  | 281,700,000 |  |  |  | 0
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  | 448,400,000 |  | 448,400,000 |  |  |  | 448,100,000
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  | 144,200,000 |  | 144,200,000 |  |  |  | 144,200,000
Notes Payable under Revolving Credit Agreement [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Line of Credit Facility, Amount Outstanding |  |  | 0 |  | 0 |  |  |  | 0
Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  | 300,000,000 |  | 300,000,000 |  |  |  | 225,000,000
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 |  | 100,000,000 |  |  |  |
Federal Home Loan Bank of Dallas [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances |  |  |  |  |  |  |  |  | 0
Federal Home Loan Bank Stock |  |  | 3,300,000 |  | 3,300,000 |  |  |  | 3,300,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances |  |  |  |  |  |  | $ 55,000,000 |  |
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 |  | 10,000,000 |  |  |  | 0
Federal Home Loan Bank Stock |  |  | 800,000 |  | 800,000 |  |  |  | 400,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances |  |  |  |  |  |  | $ 55,000,000 | $ 10,000,000 |
Federal Home Loan Bank Borrowings, Fair Value Disclosure |  |  | 10,000,000 |  | 10,000,000 |  |  |  | $ 0
Infinity [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Consideration Transferred, Liabilities Incurred | $ 110,000,000 |  |  |  |  |  |  |  |
Infinity [Member] | Secured Debt [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Consideration Transferred, Liabilities Incurred | 250,000,000 |  |  |  |  |  |  |  |
Infinity [Member] | Senior Notes [Member] | Senior Note Due on September Nineteen,Twenty Twenty Two [Member] |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Debt Instrument, Unamortized Premium | 7,100,000 |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 275,000,000 |  | $ 275,000,000 |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  | 5.00% |  | 5.00% |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt | $ (282,100,000) |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  | 4.36% |  | 4.36% |  |  |  |

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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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