Debt Obligations - Summary of Debt Repurchased a Portion of Outstanding Senior Notes on Open Market (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	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	$ 0.0	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