Fair Value Measurements - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchase of short-term investments	$ 919,000	$ 377,000
Sale of short-term investments	919,000	$ 359,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Sensitivity analysis of fair value of 10 percent change in commodity prices	2,000
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt fair value	702,900		$ 993,700
Reported Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt fair value	750,500		1,039,000
Credit facility | Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount	$ 83,300
Interest rate	1.0425%
Credit facility | Swap | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value interest rate swap	$ 400		$ 800