Acquisition and Disposition of Properties (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) a	Mar. 31, 2018 USD ($) a	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Gas and Oil Acreage [Line Items]
Payment for unproved leasehold			$ 34.2	$ 235.6
Payments for proved properties			4.8
Midland Basin
Gas and Oil Acreage [Line Items]
Number of net acres received | a	129.23
Payment for unproved leasehold			5.2	30.9
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			$ 29.0	185.9
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	240
Pre-tax gain on disposal	$ 0.7