Schedule of Quarterly Financial Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Quarterly Financial Information [Line Items]
Total revenues	$ 1,184.0		$ 1,157.7		$ 1,127.2		$ 1,121.8		$ 1,135.9		$ 1,108.0		$ 853.9		$ 833.8		$ 4,590.7	$ 3,931.6	$ 3,152.2
Income (loss) from continuing operations	(55.3)		40.9		9.8		50.7		46.4		(10.0)		(32.8)		27.9		46.1	31.5	149.6
Net income (loss)	$ (55.0)		$ 40.4		$ 20.8		$ 49.7		$ 49.6		$ (10.0)		$ (32.2)		$ 29.3		$ 55.9	$ 36.7	$ 151.2
Income (loss) from continuing operations per share, Basic	$ (1.24)		$ 0.92		$ 0.22		$ 1.13		$ 1.03		$ (0.22)		$ (0.72)		$ 0.62		$ 1.03	$ 0.70	$ 3.28
Income (loss) from continuing operations per share, Diluted	$ (1.24)	[1]	$ 0.90	[1]	$ 0.22	[1]	$ 1.11	[1]	$ 1.02	[1]	$ (0.22)	[1]	$ (0.72)	[1]	$ 0.61	[1]	$ 1.02	$ 0.69	$ 3.23
Net income (loss) per share, Basic	$ (1.24)		$ 0.91		$ 0.46		$ 1.11		$ 1.10		$ (0.22)		$ (0.71)		$ 0.65		$ 1.25	$ 0.81	$ 3.32
Net income (loss) per share, Diluted	$ (1.24)	[1]	$ 0.89	[1]	$ 0.46	[1]	$ 1.09	[1]	$ 1.09	[1]	$ (0.22)	[1]	$ (0.71)	[1]	$ 0.64	[1]	$ 1.23	$ 0.80	$ 3.27
Dividends declared per share	$ 0.33		$ 0.30		$ 0.30		$ 0.30		$ 0.300		$ 0.275		$ 0.275		$ 0.275

[1] Per diluted share amounts in the second and third quarters of 2011 and the fourth quarter of 2012 exclude common stock equivalents, since the impact of these instruments was antidilutive.