Investments - Other Investments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summarized financial information of unconsolidated entities
Total assets	$ 296,627.7				$ 276,087.8				$ 296,627.7	$ 276,087.8
Total liabilities	279,754.8				261,137.1				279,754.8	261,137.1
Total stockholders' equity	16,617.3				14,685.8				16,617.3	14,685.8	$ 11,456.0	$ 12,921.9
Total revenues	3,765.1	$ 3,310.7	$ 3,114.6	$ 4,551.3	4,047.2	$ 4,458.4	$ 3,972.6	$ 3,743.9	14,741.7	16,222.1	14,237.2
Net income (loss)	491.5	$ 235.9	$ 416.1	$ 285.0	315.3	$ 284.6	$ 392.1	$ 452.1	1,428.5	1,444.1	1,553.7
Other Types of Investments
Direct financing leases	710.8				782.9				710.8	782.9
Cash surrender value of company owned life insurance	973.6				734.1				973.6	734.1
Certain sponsored investment funds carried at fair value	638.8				678.7				638.8	678.7
Unconsolidated entities
Summarized financial information of unconsolidated entities
Total assets	155,724.1				158,439.7				155,724.1	158,439.7
Total liabilities	73,438.2				88,455.8				73,438.2	88,455.8
Total stockholders' equity	82,285.9				69,983.9				82,285.9	69,983.9
Net investment in unconsolidated entities	$ 1,912.9				$ 1,825.5				1,912.9	1,825.5
Total revenues									14,989.0	17,802.2	15,389.4
Net income (loss)									7,757.0	7,938.3	6,542.1
Our share of net income of unconsolidated entities									$ 143.9	$ 222.5	$ 161.6