Quarterly Results of Operations (Schedule of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended												12 Months Ended
	Dec. 05, 2019	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
Quarterly Financial Information Disclosure [Abstract]
Total revenues		$ 1,297.0	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9		$ 1,214.7		$ 1,198.6		$ 1,219.5		$ 4,827.3	$ 4,890.7	$ 4,494.3
Income (loss) from continuing operations		165.9	119.5	115.3	(38.7)	110.2		111.2		85.0		122.6		362.0	429.0	239.0
Net income		$ 164.6	$ 118.9	$ 115.2	$ (40.0)	$ 109.8		$ 118.9		$ 74.0		$ 122.4		$ 358.7	$ 425.1	$ 391.0
Income from continuing operations per share, Basic		$ 4.52	$ 3.17	$ 3.03	$ (1.01)	$ 2.81		$ 2.81		$ 2.09		$ 3.02		$ 9.61	$ 10.72	$ 5.63
Income from continuing operations per share, Diluted		4.47	3.14	3.01	(1.01)	2.77		2.77		2.06		2.98		9.50	10.56	5.56
Net income per share, Basic		4.49	3.15	3.03	(1.04)	2.80		3.00		1.82		3.01		9.52	10.62	9.21
Net income per share, Diluted		4.43	3.13	3.01	(1.04)	2.76		2.96		1.79		2.97		$ 9.42	$ 10.46	$ 9.09
Dividends declared per share	$ 2.50	$ 0.70	$ 0.65	$ 0.65	$ 0.65	$ 3.15	[1]	$ 0.60	[1]	$ 0.60	[1]	$ 0.60	[1]

[1] On December 5, 2019 the Board of Directors declared a special cash dividend of $2.50 per share.