Impairment (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) $ / bbl	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) $ / bbl	Jun. 30, 2020 USD ($)	Jun. 30, 2021 MMcf
Impaired Long-Lived Assets Held and Used [Line Items]
Commodity price (usd per unit)	49.78		49.78		2.43
Full cost ceiling limitation impairment	$ 0.0	$ 201.8	$ 0.0	$ 209.8
Disposal Group, Held-for-sale, Not Discontinued Operations | Corporate Headquarters Building
Impaired Long-Lived Assets Held and Used [Line Items]
Full cost ceiling limitation impairment		163.8		171.8
Impairment charge		$ 38.0		$ 38.0