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Derivative and Other Fair Value Instruments (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Derivative [Line Items] |  |  |
Secured Debt | $ 4,704,870 | $ 5,086,515 |
Proceeds from Issuance of Unsecured Debt |  | 750,000 | $ 500,000
Debt Instrument, Basis Spread on Variable Rate | 0.00% |  |
Fair Value Hedging [Member] |  |  |
Derivative [Line Items] |  |  |
Current Notional Balance | $ 450,000 |  |
Lowest Possible Notional | 450,000 |  |
Highest Possible Notional | $ 450,000 |  |
Lower Interest Rate | 2.375% |  |
Higher Interest Rate | 2.375% |  |
Earliest Maturity Date | 2019 |  |
Latest Maturity Date | 2019 |  |
Forward Starting Swaps [Member] |  |  |
Derivative [Line Items] |  |  |
Current Notional Balance | $ 50,000 |  |
Lowest Possible Notional | 50,000 |  |
Highest Possible Notional | $ 50,000 |  |
Lower Interest Rate | 2.50% |  |
Higher Interest Rate | 2.50% |  |
Earliest Maturity Date | 2026 |  |
Latest Maturity Date | 2026 |  |
2.375% Notes [Member] |  |  |
Derivative [Line Items] |  |  |
Proceeds from Issuance of Unsecured Debt | $ 450,000 | $ 450,000 |
Debt Instrument, Interest Rate, Stated Percentage | 2.375% | 2.375% |
Debt Instrument, Basis Spread on Variable Rate | 0.61% |  |
Secured Debt [Member] |  |  |
Derivative [Line Items] |  |  |
Secured Debt | $ 4,704,870 | $ 5,086,515 |
Debt Instrument, Fair Value Disclosure | 4,600,000 | 5,100,000 |
Unsecured Debt [Member] |  |  |
Derivative [Line Items] |  |  |
Unsecured Debt | 6,300,000 | 5,800,000 |
Debt Instrument, Fair Value Disclosure | $ 6,500,000 | $ 6,100,000 |
London Interbank Offered Rate (LIBOR) [Member] | 2.375% Notes [Member] |  |  |
Derivative [Line Items] |  |  |
Debt Instrument, Description of Variable Rate Basis | LIBOR plus .61% | LIBOR plus .61% |

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- Definition Highest possible notional amount of all derivatives designated as fair value or cash flow hedging instruments. The notional amount relates to a number of currency units, shares, bushels, pounds or other units specified in a derivative instrument. + References No definition available. + Details
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- Definition Lowest possible notional amount of all derivatives designated as fair value or cash flow hedging instruments. The notional amount relates to a number of currency units, shares, bushels, pounds or other units specified in a derivative instrument. + References No definition available. + Details
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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13476-108611 + Details
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- Definition Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 2: http://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 + Details
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