Debt Obligations - Summary of Debt Repurchased Portion of Outstanding Senior Notes on the Open Market (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Jun. 30, 2019
Debt Instrument [Line Items]
Debt Repurchased, Book Value	$ 303.3	$ 303.3
Debt Repurchased, Payment	(239.8)	(239.8)
Debt Repurchased, Gain		63.5
Debt Repurchased, Write-off of Debt Issuance Costs		(2.4)
Debt Repurchased, Net Gain	21.8	61.1	$ (1.4)
5⅛% Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Debt Repurchased, Book Value	19.0	19.0
Debt Repurchased, Payment	(14.6)	(14.6)
Debt Repurchased, Gain		4.4
Debt Repurchased, Write-off of Debt Issuance Costs		(0.1)
Debt Repurchased, Net Gain		4.3
5⅞% Senior Notes due 2026 [Member]
Debt Instrument [Line Items]
Debt Repurchased, Book Value	36.8	36.8
Debt Repurchased, Payment	(29.7)	(29.7)
Debt Repurchased, Gain		7.1
Debt Repurchased, Write-off of Debt Issuance Costs		(0.2)
Debt Repurchased, Net Gain		6.9
5⅜% Senior Notes due 2027 [Member]
Debt Instrument [Line Items]
Debt Repurchased, Book Value	31.9	31.9
Debt Repurchased, Payment	(26.6)	(26.6)
Debt Repurchased, Gain		5.3
Debt Repurchased, Write-off of Debt Issuance Costs		(0.2)
Debt Repurchased, Net Gain		5.1
6½% Senior Notes due 2027 [Member]
Debt Instrument [Line Items]
Debt Repurchased, Book Value	44.8	44.8
Debt Repurchased, Payment	(35.5)	(35.5)
Debt Repurchased, Gain		9.3
Debt Repurchased, Write-off of Debt Issuance Costs		(0.4)
Debt Repurchased, Net Gain		8.9
5% Senior Notes due 2028 [Member]
Debt Instrument [Line Items]
Debt Repurchased, Book Value	49.7	49.7
Debt Repurchased, Payment	(38.0)	(38.0)
Debt Repurchased, Gain		11.7
Debt Repurchased, Write-off of Debt Issuance Costs		(0.4)
Debt Repurchased, Net Gain		11.3
6⅞% Senior Notes due 2029 [Member]
Debt Instrument [Line Items]
Debt Repurchased, Book Value	70.7	70.7
Debt Repurchased, Payment	(55.2)	(55.2)
Debt Repurchased, Gain		15.5
Debt Repurchased, Write-off of Debt Issuance Costs		(0.6)
Debt Repurchased, Net Gain		14.9
5½% Senior Notes due 2030 [Member]
Debt Instrument [Line Items]
Debt Repurchased, Book Value	50.4	50.4
Debt Repurchased, Payment	$ (40.2)	(40.2)
Debt Repurchased, Gain		10.2
Debt Repurchased, Write-off of Debt Issuance Costs		(0.5)
Debt Repurchased, Net Gain		$ 9.7