Activity in Allowance for Credit Losses (Detail) - Commercial Real Estate Portfolio Segment - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses
Balance at Beginning of Year	$ 0.0	$ 1.5	$ 1.5
Provisions	0.0	0.5	3.0
Charge-offs, Net of Recoveries	0.0	(2.0)	(3.0)
Balance at End of Period	$ 0.0	$ 0.0	$ 1.5