Fair Value Measurements (Details 3) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Secured Debt | Significant Other Observable Inputs (Level 2) | Reported Value Measurement | DBJ Term Loan
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	$ 120,000	$ 120,000
Secured Debt | Significant Other Observable Inputs (Level 2) | Reported Value Measurement | ECA Term Financings
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	227,491	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	634,898	933,541
Secured Debt | Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement | DBJ Term Loan
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	120,000	120,000
Secured Debt | Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement | ECA Term Financings
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	232,030	316,285
Secured Debt | Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement | Bank Financings
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	634,132	925,783
Unsecured Debt | Quoted Prices In Active Markets for Identical Assets (Level 1) | Reported Value Measurement | Revolving Credit Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	175,000	0
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 | Senior Notes
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	$ 3,367,245	$ 3,387,125