Business and Significant Accounting Policies - Supplemental Cash Flow Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Interest Expenses [Line Items]
Senior unsecured fixed rate notes	$ 2.1	$ 0	$ 0
Other indebtedness	3.1	3.5	3.1
Junior subordinated debentures	21.3	18.3	19.3
Revolving credit facility	0	0	0.1
Total interest paid	$ 26.5	$ 21.8	$ 22.5