Share-Based Compensation, Employee Stock Purchase Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended						6 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Employee Stock [Member]	Dec. 31, 2011 Employee Stock [Member]	Dec. 31, 2010 Employee Stock [Member]	Jun. 30, 2010 January 2010 offering ESPP [Member]		Dec. 31, 2010 July 2010 offering ESPP [Member]		Jun. 30, 2011 January 2011 offering ESPP [Member]		Dec. 31, 2011 July 2011 offering ESPP [Member]		Jun. 30, 2012 January 2012 offering ESPP [Member]		Dec. 31, 2012 July 2012 offering ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized				4,000,000
Maximum employee contribution rate				5.00%
Employee stock purchase plan, purchase price				90.00%
Employee stock purchase plan, discount in grant price				10.00%
Exercised, purchase price							$ 29.63		$ 32.49		$ 43.76		$ 54.55		$ 61.84		$ 61.73
Exercised, shares issued	141,728	158,440	480,514				30,670		32,220		27,170		22,760		21,559		23,099
Proceeds received from employees							$ 0.9	[1]	$ 1.0	[1]	$ 1.2	[1]	$ 1.2	[1]	$ 1.3	[1]	$ 1.4	[1]
Expected dividend yield				0.59%	0.00%	0.00%
Expected volatility				28.97%	22.72%	31.00%
Risk-free interest rate				0.14%	0.18%	0.21%
Expected term (years)				6 months	6 months	6 months
Weighted-average grant date fair value				$ 12.40	$ 8.87	$ 6.46
Compensation expense				$ 0.5	$ 0.4	$ 0.4
Shares available for future grants				3,800,000

[1] Represents amounts received from employees through payroll deductions for share purchases under applicable offering.