Unaudited Quarterly Financial Data - Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 369.1		$ 371.0		$ 373.1		$ 371.4		$ 412.1		$ 371.3		$ 375.2		$ 360.1		$ 1,484.6		$ 1,518.7		$ 1,475.1
Net income before income taxes	44.2		36.2		34.7		62.4		80.3		48.3		44.7		42.7		177.5		216.0		179.7
Net income	$ 41.2		$ 35.3		$ 27.9		$ 58.8		$ 59.7		$ 44.7		$ 38.6		$ 40.2		$ 163.2		$ 183.2		$ 143.2
Net income per common share:
Basic	$ 1.48	[1]	$ 1.27	[1]	$ 1.00	[1]	$ 2.09	[1]	$ 2.11	[1]	$ 1.57	[1]	$ 1.34	[1]	$ 1.38	[1]	$ 5.84	[1]	$ 6.39	[1]	$ 4.85
Diluted	$ 1.44	[1]	$ 1.24	[1]	$ 0.98	[1]	$ 2.05	[1]	$ 2.07	[1]	$ 1.54	[1]	$ 1.32	[1]	$ 1.36	[1]	$ 5.72	[1]	$ 6.27	[1]	$ 4.67
Comprehensive income (loss)	$ 29.2		$ (18.3)		$ 18.9		$ 36.8		$ 31.3		$ 6.8		$ 64.1		$ 41.3		$ 66.6		$ 143.5		$ 101.5

[1] Basic and diluted net income per common share are computed independently for each quarter and full year based on the respective average number of common shares outstanding; therefore, the sum of the quarterly net income per common share data may not equal the net income per common share for the year.