Quarterly Results of Operations (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Results of Operations (Unaudited)
Total revenues	$ 3,237.7	$ 4,627.8	$ 3,178.3	$ 3,049.4	$ 3,513.8	$ 2,818.0	$ 3,025.7	$ 3,036.6	$ 14,093.2	$ 12,394.1	$ 11,964.4
Total expenses	2,922.9	3,470.0	2,812.7	2,635.4	3,132.5	2,447.3	2,625.8	2,596.8	11,841.0	10,802.4	10,533.6
Net income (loss)	842.9	813.2	314.8	353.6	338.2	327.4	327.0	369.2	2,324.5	1,361.8	1,253.2
Net income (loss) available to common stockholders	$ 841.8	$ 810.2	$ 309.5	$ 348.9	$ 318.0	$ 308.2	$ 322.3	$ 368.0	$ 2,310.4	$ 1,316.5	$ 1,209.3
Basic earnings per common share (in dollars per share)	$ 2.91	$ 2.80	$ 1.07	$ 1.21	$ 1.10	$ 1.07	$ 1.11	$ 1.26	$ 8.00	$ 4.55	$ 4.11
Diluted earnings per common share (in dollars per share)	$ 2.87	$ 2.76	$ 1.06	$ 1.19	$ 1.09	$ 1.06	$ 1.10	$ 1.25	$ 7.88	$ 4.50	$ 4.06