Fair Value Measurements (Details 3) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Secured Debt | Significant Other Observable Inputs (Level 2) | Reported Value Measurement | ECA Term Financings {Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	$ 236,879	$ 305,276
Secured Debt | Significant Other Observable Inputs (Level 2) | Reported Value Measurement | Bank Financings
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	651,434	933,541
Secured Debt | Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement [Member] | ECA Term Financings {Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	243,880	316,285
Secured Debt | Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement [Member] | Bank Financings
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	649,557	925,783
Unsecured Debt | Quoted Prices in Active Markets for Identical Assets (Level 1) | Reported Value Measurement | Revolving Credit Facility [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	120,000	120,000
Unsecured Debt | Quoted Prices in Active Markets for Identical Assets (Level 1) | Reported Value Measurement | Senior Notes
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	3,200,000	3,200,000
Unsecured Debt | Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimate of Fair Value Measurement [Member] | Senior Notes
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	$ 3,422,824	$ 3,387,125