Gross Realized Gains and Losses on Sales and OTTI on Securities Available for Sale (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale Securities, Gross Realized Gains	$ 4.0	$ 24.5	$ 28.0
Available-for-sale Securities	30,672.0	27,825.0
Available-for-sale Securities, Gross Realized Losses	0.0	(3.0)	(42.0)
Gain (Loss) on Sale of Securities, Net	(4.0)	(21.0)	15.0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	$ 0.0	$ (1.0)	$ (1.0)