Other Long-term Liabilities, Mandatorily Redeemable Preferred Interests (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) JointVenture	Jun. 30, 2016 USD ($)
Changes in long-term liabilities attributable to mandatorily redeemable preferred interests [Abstract]
Balance at December 31, 2016	$ 68.5
Change in estimated redemption value included in interest expense	6.9	$ (14.6)
Balance at June 30, 2017	77.2
Mandatorily Redeemable Noncontrolling Interests [Member]
Changes in long-term liabilities attributable to mandatorily redeemable preferred interests [Abstract]
Balance at December 31, 2016	68.5
Income attributable to mandatorily redeemable preferred interests	1.8
Change in estimated redemption value included in interest expense	6.9
Balance at June 30, 2017	$ 77.2
Mandatorily Redeemable Noncontrolling Interests [Member] | West OK [Member]
Changes in long-term liabilities attributable to mandatorily redeemable preferred interests [Abstract]
Ownership interest	100.00%
Mandatorily Redeemable Noncontrolling Interests [Member] | West TX [Member]
Changes in long-term liabilities attributable to mandatorily redeemable preferred interests [Abstract]
Ownership interest	72.80%
Mandatorily Redeemable Noncontrolling Interests [Member] | Joint Ventures [Member]
Changes in long-term liabilities attributable to mandatorily redeemable preferred interests [Abstract]
Number of joint ventures | JointVenture	2