Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 9,770.4	$ 8,705.9
Net Change	(44.3)	2.7	$ (53.0)
Balance, end of period	10,216.2	9,770.4	8,705.9
Net Unrealized (Losses) Gains on Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(32.4)	(31.0)	27.6
Net Change	(42.4)	(1.4)	(58.6)
Balance, end of period	(74.8)	(32.4)	(31.0)
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	6.1	(3.0)	(4.7)
Net Change	(1.6)	9.1	1.7
Balance, end of period	4.5	6.1	(3.0)
Net Foreign Currency Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(18.5)	(17.6)	(1.7)
Net Change	16.7	(0.9)	(15.9)
Balance, end of period	(1.8)	(18.5)	(17.6)
Net Pension and Other Postretirement Benefit Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(325.2)	(321.1)	(340.9)
Net Change	(17.0)	(4.1)	19.8
Balance, end of period	(342.2)	(325.2)	(321.1)
ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(370.0)	(372.7)	(319.7)
Net Change	(44.3)	2.7	(53.0)
Balance, end of period	$ (414.3)	$ (370.0)	$ (372.7)