Acquisition and Disposition of Properties (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) a	Mar. 31, 2017 USD ($)
Gas and Oil Acreage [Line Items]
Payment for unproved leasehold	$ 21.8	$ 157.8
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	18.3	111.9
Midland Basin
Gas and Oil Acreage [Line Items]
Payment for unproved leasehold	$ 3.5	27.1
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