Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Numerator:
Income from continuing operations	$ 60.2	$ 66.4	$ 64.3	$ 44.3	$ 44.0	$ 64.4	$ 76.1	$ 44.2	$ 235.2		$ 228.7		$ 172.0
Less: dividends declared - common stock outstanding, unvested restricted shares and restricted share units									(53.7)		(48.0)		(43.5)
Undistributed earnings									181.5		180.7		128.5
Percent allocated to common shareholders									99.50%		99.40%		99.00%
Undistributed earnings allocated to common shareholders									180.6		179.7		127.3
Add: dividends declared - common stock									53.4		47.7		43.1
Numerator for basic and diluted EPS									234.0		227.4		170.4
Denominator (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding									63,471,000		62,513,000		61,457,000
Effect of dilutive securities:
Performance awards (in shares)									416,000		399,000		318,000
Stock options (in shares)									919,000		698,000		720,000
Denominator for diluted EPS: adjusted weighted average common shares outstanding and assumed conversion									64,806,000		63,610,000		62,495,000
Per share income from continuing operations:
Basic (in dollars per share)	$ 0.94	$ 1.04	$ 1.01	$ 0.70	$ 0.69	$ 1.02	$ 1.21	$ 0.71	$ 3.69	[1]	$ 3.64	[1]	$ 2.77	[1]
Diluted (in dollars per share)	$ 0.92	$ 1.02	$ 0.99	$ 0.68	$ 0.68	$ 1.00	$ 1.19	$ 0.69	$ 3.61	[1]	$ 3.57	[1]	$ 2.73	[1]
Basic weighted-average common shares outstanding									63,471,000		62,513,000		61,457,000
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units									63,797,000		62,871,000		62,047,000
Percent allocated to common shareholders									99.50%		99.40%		99.00%
Income (loss) from discontinued operations and net income
Income (loss) from discontinued operations attributable to common shareholders for basic and diluted earnings per share									(25.4)		41.2		8.2
Net income attributable to common shareholders for basic and diluted earnings per share									$ 208.6		$ 268.6		$ 178.6
Antidilutive stock options excluded from EPS calculation (in shares)													200,000

[1] Earning per share calculated based on the two-class method. See Note 8 for detailed calculations.