Derivative Financial Instruments (Notional Amounts, Balance Sheet Location And Estimated Fair Values Of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Contractual notional amount	$ 2,048.1	$ 16,834.9
Total derivative assets	3.1	758.7
Amounts netted	(1.6)	(92.8)
Net derivative assets	1.5	665.9
Total derivative liabilities	(21.5)	(241.9)
Amounts netted	1.6	92.8
Net derivative liabilities	(19.9)	(149.1)
Hedging Instruments [Member]
Derivative [Line Items]
Contractual notional amount	1,235.0	1,385.0
Hedging Instruments [Member] | Other Assets/Other Liabilities
Derivative [Line Items]
Contractual notional amount	1,660.0	6,160.0
Total derivative assets	2.0	532.8
Total derivative liabilities	(14.2)	(6.7)
Hedging Instruments [Member] | Other Assets/Other Liabilities | Interest Rate Swaps [Member]
Derivative [Line Items]
Contractual notional amount	1,235.0	1,385.0
Total derivative assets	2.0	4.0
Total derivative liabilities	(5.3)	(0.7)
Hedging Instruments [Member] | Other Assets/Other Liabilities | Forward Contracts [Member]
Derivative [Line Items]
Contractual notional amount	425.0	4,775.0
Total derivative assets	0.0	528.8
Total derivative liabilities	(8.9)	(6.0)
Non-Hedging Instruments [Member]
Derivative [Line Items]
Contractual notional amount	388.1	10,674.9
Total derivative assets	1.1	225.9
Total derivative liabilities	(7.3)	(235.2)
Non-Hedging Instruments [Member] | Other Assets/Other Liabilities | Options [Member]
Derivative [Line Items]
Contractual notional amount	100.0	10,280.2
Total derivative assets	0.0	220.7
Total derivative liabilities	(0.1)	(233.9)
Non-Hedging Instruments [Member] | Equity Securities [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Contractual notional amount	171.3	209.4
Total derivative assets	1.0	4.7
Total derivative liabilities	(4.7)	(0.2)
Non-Hedging Instruments [Member] | Equity Securities [Member] | Futures [Member]
Derivative [Line Items]
Contractual notional amount	116.8	185.3
Total derivative assets	0.1	0.5
Total derivative liabilities	$ (2.5)	$ (1.1)