Acquisition and Disposition of Properties (Details) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018 USD ($) a	Jun. 30, 2018 USD ($) a	Mar. 31, 2018 USD ($) a	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Gas and Oil Acreage [Line Items]
Payment for unproved leasehold				$ 75.3	$ 259.3
Payments for proved properties				5.8
Midland Basin
Gas and Oil Acreage [Line Items]
Number of net acres received | a	608	129.23
Payment for unproved leasehold				7.7	32.4
Delaware Basin
Gas and Oil Acreage [Line Items]
Number of net acres received | a			1,230.7
Cash received in acquiring acreage in unproved leasehold properties			$ 0.7
Payment for unproved leasehold				$ 67.7	208.1
Delaware Basin - Mineral Purchases
Gas and Oil Acreage [Line Items]
Payment for unproved leasehold					$ 18.8
Midland Basin
Gas and Oil Acreage [Line Items]
Number of net acres delivered | a			1,922.4
Pre-tax gain on disposal			$ 33.4
Central Basin Platform
Gas and Oil Acreage [Line Items]
Number of net acres delivered | a	612	240
Pre-tax gain on disposal	$ 0.1	$ 0.7