Activity in Allowance for Credit Losses (Detail) (Commercial Real Estate Portfolio Segment, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commercial Real Estate Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Balance at Beginning of Year	$ 1.5	$ 1.5	$ 1.5
Provisions	3.0	0	1.8
Charge-offs, Net of Recoveries	(3.0)	0	(1.8)
Balance at End of Period	$ 1.5	$ 1.5	$ 1.5