Quarterly Operating Results (Unaudited) - Quarterly Operating Results (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Total revenues	$ 1,245.4	$ 1,286.8	$ 1,179.3	$ 915.0	$ 890.2	$ 835.8	$ 779.5	$ 674.1	$ 4,626.5	$ 3,179.6	$ 2,520.3
Total expenses	1,219.1	1,199.1	1,072.2	868.7	840.9	750.3	682.1	631.8	(4,359.1)	(2,905.1)	(2,275.0)
Earnings before income taxes	26.3	87.7	107.1	46.3	49.3	85.5	97.4	42.3	267.4	274.5	245.3
Net earnings	$ 51.5	$ 93.6	$ 109.0	$ 49.3	$ 60.0	$ 74.6	$ 93.5	$ 40.5	$ 303.4	$ 268.6	$ 195.0
Basic net earnings per share:	$ 0.32	$ 0.58	$ 0.71	$ 0.37	$ 0.45	$ 0.57	$ 0.73	$ 0.32
Diluted net earnings per share:	$ 0.31	$ 0.58	$ 0.70	$ 0.36	$ 0.45	$ 0.57	$ 0.73	$ 0.32