Quarterly Results of Operations (Unaudited) - 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 Results of Operations (Unaudited)
Total revenues	$ 2,807.0	$ 3,240.9	$ 3,259.2	$ 2,657.3	$ 2,908.0	$ 2,484.3	$ 2,606.7	$ 2,478.6	$ 11,964.4	$ 10,477.6	$ 9,289.5
Total expenses	2,498.0	2,871.7	2,964.7	2,199.2	2,590.8	2,090.5	2,199.2	2,102.2	10,533.6	8,982.7	8,165.5
Net income (loss)	258.3	300.9	264.9	429.1	280.3	252.8	319.2	324.1	1,253.2	1,176.4	936.1
Net income (loss) available to common stockholders	$ 253.6	$ 300.4	$ 241.1	$ 414.2	$ 270.4	$ 240.7	$ 306.3	$ 293.7	$ 1,209.3	$ 1,111.1	$ 879.7
Basic earnings per common share (in dollars per share)	$ 0.87	$ 1.02	$ 0.82	$ 1.41	$ 0.92	$ 0.78	$ 1.04	$ 0.96	$ 4.11	$ 3.70	$ 2.99
Diluted earnings per common share (in dollars per share)	$ 0.86	$ 1.01	$ 0.81	$ 1.39	$ 0.91	$ 0.77	$ 1.03	$ 0.95	$ 4.06	$ 3.65	$ 2.95