Investments - Other Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summarized financial information of unconsolidated entities
Total assets, unconsolidated entities	$ 136,171.8	$ 123,621.2
Total liabilities, unconsolidated entities	83,815.2	73,688.8
Total equity, unconsolidated entities	52,356.6	49,932.4
Net investment in unconsolidated entities	1,616.2	1,223.8
Total revenues, unconsolidated entities	15,020.9	14,376.4	$ 13,171.0
Net income, unconsolidated entities	4,530.1	3,455.3	4,866.0
Our share of net income of unconsolidated entities	232.7	232.7	$ 165.3
Other Types of Investments
Direct financing leases	$ 937.4	$ 875.2