Fair Value Measurements - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Sale of short-term investments	$ 919,000,000	$ 149,000,000
Purchase of short-term investments	919,000,000	$ 191,000,000
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	3,000,000
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt fair value	667,900,000		$ 993,700,000
Reported Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt fair value	687,000,000		1,039,000,000
Credit facility | Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount	$ 100,000,000
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	$ 500,000		$ 800,000