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. 30, 2018	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues			$ 1,257.9		$ 1,214.7		$ 1,198.6		$ 1,219.5		$ 1,102.8		$ 1,166.6		$ 1,133.4		$ 1,091.5		$ 4,890.7	$ 4,494.3	$ 4,267.9
Income from continuing operations			110.2		111.2		85.0		122.6		2.1		104.0		82.4		50.5		429.0	239.0	216.1
Net income			$ 109.8		$ 118.9		$ 74.0		$ 122.4		$ 123.6		$ 100.4		$ 99.3		$ 67.7		$ 425.1	$ 391.0	$ 186.2
Income from continuing operations per share, Basic			$ 2.81		$ 2.81		$ 2.09		$ 3.02		$ 0.05		$ 2.45		$ 1.94		$ 1.19		$ 10.72	$ 5.63	$ 5.08
Income from continuing operations per share, Diluted			2.77		2.77		2.06		2.98		0.05		2.41		1.91		1.17		10.56	5.56	5.03
Net income per share, Basic			2.80		3.00		1.82		3.01		2.92		2.36		2.34		1.59		10.62	9.21	4.38
Net income per share, Diluted			2.76		2.96		1.79		2.97		2.88		2.33		2.31		1.57		$ 10.46	$ 9.09	$ 4.33
Dividends declared per share	$ 2.50	$ 4.75	$ 3.15	[1]	$ 0.60	[1]	$ 0.60	[1]	$ 0.60	[1]	$ 5.35	[2]	$ 0.54	[2]	$ 0.54	[2]	$ 0.54	[2]

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

[2] As a result of the sale of Chaucer (see Note 2 – “Discontinued Operations”), on December 30, 2018 the Board of Directors declared a special dividend of $4.75 per share.