Investments (Collectively Significant Equity Method Investments - Narrative) (Details) - USD ($) $ in Billions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity Method Investment, Summarized Financial Information, Gross Profit (Loss)
Carrying Value of investments accounted for under the equity method		$ 15.6
Unfunded commitments for investments accounted for under the equity method		$ 6.1
Aggregate Net income Exceeded Stated Percentage Of The Pre Tax Income (Loss) From Continuing Operations	10.00%	10.00%
Total assets for investments accounted for under the equity method		$ 585.3	$ 529.1
Total liabilities for investments accounted for under the equity method		86.1	65.5
Net Income (loss) for investments accounted for under the equity method		$ 47.0	$ 52.5	$ 37.9