Derivative Instruments and Hedging Activities - Gains and losses on derivatives (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gains and losses on derivatives
Loss before provision for income taxes									$ 37.9	$ (152.4)	$ 27.0
Income tax (expense) benefit									(1.5)	65.1	158.3
Net income (loss)	$ 6.9	$ 11.4	$ 9.7	$ 8.4	$ 17.1	$ 31.4	$ 25.4	$ (161.2)	36.4	(87.3)	$ 185.3
Interest rate swaps | Hedging | Cash flow hedging
Gains and losses on derivatives
Losses on derivative									6.2	4.8
Accumulated Gain (Loss), Net, Cash Flow Hedge | Interest rate swaps | Hedging | Cash flow hedging | Reclassification out of Accumulated Other Comprehensive Income
Gains and losses on derivatives
Loss before provision for income taxes									13.8	6.5
Income tax (expense) benefit									(4.8)
Net income (loss)									$ 9.0	$ 6.5