Investments (Aggregate Fair Value And Gross Unrealized Loss Of Fixed Maturity Securities And Equity Securities In An Unrealized Loss Position) (Details) $ in Millions	Mar. 31, 2017 USD ($) securities	Dec. 31, 2016 USD ($) securities
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	2,878	3,330
Estimated fair value, less than 12 months	$ 5,567.7	$ 6,220.3
Gross unrealized loss, less than 12 months	$ (96.3)	$ (138.5)
Number of securities, 12 months or greater | securities	321	425
Estimated fair value, 12 months or greater	$ 563.0	$ 816.0
Gross unrealized loss, 12 months or greater	$ (28.0)	$ (42.4)
Equity Securities [Member]
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	450	452
Estimated fair value, less than 12 months	$ 270.8	$ 233.1
Gross unrealized loss, less than 12 months	$ (13.6)	$ (10.7)
Number of securities, 12 months or greater | securities	0	0
Estimated fair value, 12 months or greater	$ 0.0	$ 0.0
Gross unrealized loss, 12 months or greater	$ 0.0	$ 0.0
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	2,428	2,878
Estimated fair value, less than 12 months	$ 5,296.9	$ 5,987.2
Gross unrealized loss, less than 12 months	$ (82.7)	$ (127.8)
Number of securities, 12 months or greater | securities	321	425
Estimated fair value, 12 months or greater	$ 563.0	$ 816.0
Gross unrealized loss, 12 months or greater	$ (28.0)	$ (42.4)
Fixed Maturities [Member] | United States Government Securities [Member]
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	41	51
Estimated fair value, less than 12 months	$ 235.8	$ 359.9
Gross unrealized loss, less than 12 months	$ (3.0)	$ (5.7)
Number of securities, 12 months or greater | securities	0	0
Estimated fair value, 12 months or greater	$ 0.0	$ 0.0
Gross unrealized loss, 12 months or greater	$ 0.0	$ 0.0
Fixed Maturities [Member] | Government Sponsored Securities [Member]
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	17	18
Estimated fair value, less than 12 months	$ 42.9	$ 26.4
Gross unrealized loss, less than 12 months	$ (0.3)	$ (0.3)
Number of securities, 12 months or greater | securities	1	1
Estimated fair value, 12 months or greater	$ 1.0	$ 1.0
Gross unrealized loss, 12 months or greater	$ (0.1)	$ (0.1)
Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	598	1,022
Estimated fair value, less than 12 months	$ 1,086.6	$ 1,849.0
Gross unrealized loss, less than 12 months	$ (31.0)	$ (55.2)
Number of securities, 12 months or greater | securities	24	28
Estimated fair value, 12 months or greater	$ 40.8	$ 60.7
Gross unrealized loss, 12 months or greater	$ (2.1)	$ (3.2)
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	1,242	1,272
Estimated fair value, less than 12 months	$ 2,835.5	$ 2,640.6
Gross unrealized loss, less than 12 months	$ (32.4)	$ (49.5)
Number of securities, 12 months or greater | securities	137	203
Estimated fair value, 12 months or greater	$ 305.7	$ 422.8
Gross unrealized loss, 12 months or greater	$ (15.8)	$ (27.1)
Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	455	430
Estimated fair value, less than 12 months	$ 920.1	$ 905.8
Gross unrealized loss, less than 12 months	$ (15.1)	$ (15.3)
Number of securities, 12 months or greater | securities	101	114
Estimated fair value, 12 months or greater	$ 101.1	$ 136.9
Gross unrealized loss, 12 months or greater	$ (4.1)	$ (4.6)
Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	19	19
Estimated fair value, less than 12 months	$ 53.7	$ 61.2
Gross unrealized loss, less than 12 months	$ (0.3)	$ (0.3)
Number of securities, 12 months or greater | securities	19	24
Estimated fair value, 12 months or greater	$ 30.0	$ 60.8
Gross unrealized loss, 12 months or greater	$ (2.9)	$ (3.4)
Fixed Maturities [Member] | Other Securities [Member]
Schedule of Investments [Line Items]
Number of securities, less than 12 months | securities	56	66
Estimated fair value, less than 12 months	$ 122.3	$ 144.3
Gross unrealized loss, less than 12 months	$ (0.6)	$ (1.5)
Number of securities, 12 months or greater | securities	39	55
Estimated fair value, 12 months or greater	$ 84.4	$ 133.8
Gross unrealized loss, 12 months or greater	$ (3.0)	$ (4.0)