Long-Term Debt - Other Long-Term Debt (Details) - Wells Fargo Bank N.A. - Line of credit $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Aug. 01, 2017 USD ($)	Aug. 01, 2016 USD ($)	Dec. 31, 2009 agreement
Other Long-Term Debt
Number of credit agreements | agreement							2
Revolving credit facility | Revolver
Other Long-Term Debt
Borrowing base						$ 425.0
Cash balance threshold						$ 30.0
Average interest rate (as a percent)	2.84%	2.25%	2.72%	2.43%
Average outstanding balance	$ 194.6	$ 185.0	$ 194.9	$ 164.0
Total interest and commitment fees	$ 1.6	$ 1.3	$ 3.1	$ 2.6
Covenant, total leverage ratio	4.0		4.0
Covenant, current ratio	1.00		1.00
Total leverage ratio	3.84		3.84
Current ratio	2.70		2.70
Revolving credit facility | London Interbank Offered Rate (LIBOR) | Minimum | Revolver
Other Long-Term Debt
Margin interest rate (as a percent)			1.50%
Revolving credit facility | London Interbank Offered Rate (LIBOR) | Maximum | Revolver
Other Long-Term Debt
Margin interest rate (as a percent)			2.50%
Revolving credit facility | Federal Funds Effective Swap Rate | Revolver
Other Long-Term Debt
Margin interest rate (as a percent)			0.50%
Revolving credit facility | One Month Adjust LIBO Rate | Revolver
Other Long-Term Debt
Margin interest rate (as a percent)			1.00%
Revolving credit facility | Base rate | Minimum | Revolver
Other Long-Term Debt
Margin interest rate (as a percent)			0.50%
Revolving credit facility | Base rate | Maximum | Revolver
Other Long-Term Debt
Margin interest rate (as a percent)			1.50%
Subsequent Event | Revolving credit facility | Revolver
Other Long-Term Debt
Borrowing base					$ 375.0