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Debt - Notes - Additional Information (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Debt Instrument [Line Items] |  |  |
Repayments of notes payable | $ 497,975 | $ 1,500,000 | $ 300,000
Unsecured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Write off of deferred debt issuance cost |  | 1,900 |
Write off unamortized (premium) discount |  | 25,200 |
5.75 % Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Repayments of notes payable | $ 394,100 |  |
Debt instrument, interest rate, stated percentage | 5.75% |  |
7.125% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Repayments of notes payable | $ 103,900 | $ 46,100 |
Debt instrument, interest rate, stated percentage | 7.125% | 7.125% |
Prepayment penalty charges |  | $ 4,600 |
3.25% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument, interest rate, stated percentage | 3.25% |  |
Proceeds from issuance of unsecured debt | $ 400,000 |  |
Proceeds from debt, net of issuance costs | $ 399,300 |  |
Debt instrument, interest rate, effective percentage | 3.32% |  |
4.00% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument, interest rate, stated percentage | 4.00% |  |
Proceeds from issuance of unsecured debt | $ 300,000 |  |
Proceeds from debt, net of issuance costs | $ 293,200 |  |
Debt instrument, interest rate, effective percentage | 4.11% |  |
5.125% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Repayments of notes payable |  | $ 228,900 |
Debt instrument, interest rate, stated percentage |  | 5.125% |
Prepayment penalty charges |  | $ 1,400 |
Repayments of notes payable at maturity |  | 271,100 |
5.375% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Repayments of notes payable |  | $ 400,000 |
Debt instrument, interest rate, stated percentage |  | 5.375% |
Prepayment penalty charges |  | $ 9,500 |
5.750% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Repayments of notes payable |  | $ 255,900 |
Debt instrument, interest rate, stated percentage |  | 5.75% |
Prepayment penalty charges |  | $ 16,500 |
4.625% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Repayments of notes payable |  | $ 250,000 |
Debt instrument, interest rate, stated percentage |  | 4.625% |
Prepayment penalty charges |  | $ 31,600 |
7.570% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Repayments of notes payable |  | $ 48,000 |
Debt instrument, interest rate, stated percentage |  | 7.57% |
Prepayment penalty charges |  | $ 19,300 |
2.85% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument, interest rate, stated percentage |  | 2.85% |
Proceeds from issuance of unsecured debt |  | $ 500,000 |
Proceeds from debt, net of issuance costs |  | $ 496,700 |
Debt instrument, interest rate, effective percentage |  | 3.10% |

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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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