Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	$ (319.9)	$ (224.0)	$ (59.5)
Other comprehensive income (loss), before reclassifications	(42.7)	(92.5)	6.5
Amounts reclassified from accumulated other comprehensive income (loss)	44.2	(3.4)	(171.0)
Net current period other comprehensive income (loss)	1.5	(95.9)	(164.5)
Accumulated other comprehensive income (loss), net of tax ending balance	(318.4)	(319.9)	(224.0)
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(10.8)	(0.8)	(0.4)
Other comprehensive income (loss), before reclassifications	(3.5)	(10.6)	(1.7)
Amounts reclassified from accumulated other comprehensive income (loss)	12.7	0.6	1.3
Net current period other comprehensive income (loss)	9.2	(10.0)	(0.4)
Accumulated other comprehensive income (loss), net of tax ending balance	(1.6)	(10.8)	(0.8)
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	57.8	10.2	71.7
Other comprehensive income (loss), before reclassifications	(193.8)	51.6	110.8
Amounts reclassified from accumulated other comprehensive income (loss)	31.5	(4.0)	(172.3)
Net current period other comprehensive income (loss)	(162.3)	47.6	(61.5)
Accumulated other comprehensive income (loss), net of tax ending balance	(104.5)	57.8	10.2
Unfunded Status of Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(32.7)	(37.8)	(31.6)
Other comprehensive income (loss), before reclassifications	(4.1)	5.1	(6.2)
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0	0.0
Net current period other comprehensive income (loss)	(4.1)	5.1	(6.2)
Accumulated other comprehensive income (loss), net of tax ending balance	(36.8)	(32.7)	(37.8)
Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(334.2)	(195.6)	(99.2)
Other comprehensive income (loss), before reclassifications	158.7	(138.6)	(96.4)
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0	0.0
Net current period other comprehensive income (loss)	158.7	(138.6)	(96.4)
Accumulated other comprehensive income (loss), net of tax ending balance	$ (175.5)	$ (334.2)	$ (195.6)