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Credit Facility and Long-Term Debt - Additional Information (Details) - USD ($) |  |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Mar. 27, 2017 | Nov. 13, 2013 | Dec. 15, 2010 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2014 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 30, 2016
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Revolving credit facility, credit agreement maximum limit | $ 200,000,000 |  |  |  |  |  |  |  |  |
Line of credit facility Increasable capacity | $ 100,000,000 |  |  |  |  |  |  |  |  |
Line of credit facility borrowings outstanding |  |  |  |  |  |  | $ 0 |  |  |
Minimum consolidated tangible net worth under credit facility |  |  |  |  |  |  | $ 2,323,100,000.0 |  |  |
Percentage of consolidated net income |  |  |  |  |  |  | 25.00% |  |  |
Percentage of aggregate net cash proceeds from the issuance of capital stock |  |  |  |  |  |  | 25.00% |  |  |
Percentage of consolidated leverage ratio permitted |  |  |  |  |  |  | 35.00% |  |  |
Payment for extinguishment of debt |  |  |  |  |  |  | $ 8,600,000 | $ 0 | $ 0 |
Long-term debt repayment |  |  |  |  |  |  | 125,000,000 | 0 | $ 0 |
Citibank Europe |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Outstanding collateralized letters of credit facility |  |  |  |  |  |  | $ 444,200,000 | 449,400,000 |  |
London Interbank Offered Rate (LIBOR) |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Interest Rate During Period |  |  |  |  |  |  | 1.00% |  |  |
Citibank Europe plc | Letters of credit / guarantees |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Revolving credit facility, credit agreement maximum limit |  |  |  |  |  |  |  |  |  | $ 550,000,000.0
Senior Notes |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Loans issued |  | $ 300,000,000.0 | $ 250,000,000.0 |  |  |  |  |  |  |
Debt stated interest rate |  | 4.65% | 6.00% |  |  |  |  |  |  |
Net proceeds from senior notes |  | $ 299,700,000 | $ 247,500,000 |  |  |  |  |  |  |
Amount of indebtedness subject to default in payment |  |  |  |  |  |  | $ 50,000,000.0 |  |  |
Annual interest payable |  |  |  |  |  |  | 21,500,000 |  |  |
Silverton 2015 |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Loan notes issued by third parties |  |  |  |  |  | $ 85,000,000 |  |  |  |
Silverton 2015, Third-party Funded |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Loan notes issued by third parties |  |  |  |  |  | $ 70,000,000 |  |  |  |
Silverton 2016 |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Loan notes issued by third parties |  |  |  |  | $ 125,000,000 |  |  |  |  |
Silverton 2017 |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Loan notes issued by third parties |  |  |  | $ 130,000,000 |  |  |  |  |  |
Silverton 2017 | Silverton Loan Notes | Aspen Holdings |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum exposure to loss |  |  |  |  |  |  | $ 1,100,000 |  |  |
Silverton 2017, Third-party Funded |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Loan notes issued by third parties |  |  |  |  |  |  |  | $ 105,000,000 |  |
Silverton 2016, Third-party Funded |  |  |  |  |  |  |  |  |  |
Line Of Credit Facility Covenant Compliance [Line Items] |  |  |  |  |  |  |  |  |  |
Loan notes issued by third parties |  |  |  |  | $ 100,000,000 |  |  |  |  |

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- Definition Line Of Credit Facility Covenant Compliance [Line Items] + References No definition available. + Details
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- Definition Line of credit facility Increasable capacity. + References No definition available. + Details
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- Definition Line of credit facility increase additional borrowing capacity. + References No definition available. + Details
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- Definition Line of credit facility tangible net worth cash proceeds from the issuance of capital stock restriction. + References No definition available. + Details
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- Definition Line of credit facility tangible net worth net income restriction. + References No definition available. + Details
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- Definition Line of credit facility tangible net worth restrictions minimum. + References No definition available. + Details
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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: 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(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: 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 2: 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 Reference 3: 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 + Details
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- Definition The effective interest rate during the reporting period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of the required periodic payment applied to interest. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The fair value of notes issued in noncash investing and financing activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
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- Definition The cash inflow from a borrowing with the highest claim on the assets of the entity in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle, if longer). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585 + Details
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