Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			$ (318.4)	$ (319.9)
Balance, as of January 1, 2018	$ (261.6)	$ (338.8)	(318.4)	(319.9)	$ (316.9)
Other comprehensive income (loss) before reclassifications			7.2	(15.4)
Amounts reclassified from accumulated other comprehensive income (loss)			48.1	(3.5)
Net current period other comprehensive income (loss)	42.3	(13.6)	55.3	(18.9)
Balance, as of the end of the period	(261.6)	(338.8)	(261.6)	(338.8)
Accumulated net unrealized gains (losses) on securities available-for-sale
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(1.6)	(10.8)
Balance, as of January 1, 2018	(1.7)	(5.2)	(1.6)	(10.8)	(0.1)
Other comprehensive income (loss) before reclassifications			(7.6)	5.1
Amounts reclassified from accumulated other comprehensive income (loss)			6.0	0.5
Net current period other comprehensive income (loss)			(1.6)	5.6
Balance, as of the end of the period	(1.7)	(5.2)	(1.7)	(5.2)
Accumulated net unrealized gains (losses) on cash flow hedges
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(104.5)	57.8
Cumulative effect of new accounting principle					0.0
Balance, as of January 1, 2018	(0.7)	(69.0)	(104.5)	57.8	(104.5)
Other comprehensive income (loss) before reclassifications			61.7	(122.8)
Amounts reclassified from accumulated other comprehensive income (loss)			42.1	(4.0)
Net current period other comprehensive income (loss)			103.8	(126.8)
Balance, as of the end of the period	(0.7)	(69.0)	(0.7)	(69.0)
Accumulated unfunded status of postretirement benefit plans
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(36.8)	(32.7)
Cumulative effect of new accounting principle					0.0
Balance, as of January 1, 2018	(36.4)	(33.2)	(36.8)	(32.7)	(36.8)
Other comprehensive income (loss) before reclassifications			0.4	(0.5)
Amounts reclassified from accumulated other comprehensive income (loss)			0.0	0.0
Net current period other comprehensive income (loss)			0.4	(0.5)
Balance, as of the end of the period	(36.4)	(33.2)	(36.4)	(33.2)
Accumulated translation adjustment
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(175.5)	(334.2)
Cumulative effect of new accounting principle					0.0
Balance, as of January 1, 2018	(222.8)	(231.4)	(175.5)	(334.2)	(175.5)
Other comprehensive income (loss) before reclassifications			(47.3)	102.8
Amounts reclassified from accumulated other comprehensive income (loss)			0.0	0.0
Net current period other comprehensive income (loss)			(47.3)	102.8
Balance, as of the end of the period	$ (222.8)	$ (231.4)	$ (222.8)	$ (231.4)
Accounting Standards Update 2016-01
Accumulated other comprehensive income [Line Items]
Cumulative effect of new accounting principle					1.5
Accounting Standards Update 2016-01 | Accumulated net unrealized gains (losses) on securities available-for-sale
Accumulated other comprehensive income [Line Items]
Cumulative effect of new accounting principle					$ 1.5