Debt Commitments and Contingencies - Debt Obligations and Undrawn Letters of Credit Outstanding (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Total debt	$ 365,000	$ 90,000
Less: Current portion of debt	(62,050)	(90,000)
Long-term debt	302,950	0
Credit Facility [Member] | Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Total debt	350,000	90,000
Credit Facility [Member] | Letter of Credit Subfacility [Member]
Debt Instrument [Line Items]
Undrawn letters of credit under unsecured revolving credit facility - face amount	36,266	37,315
Note [Member]
Debt Instrument [Line Items]
Total debt	$ 15,000	$ 0