Common Stock Incentive Plan (Details) (USD $) In Millions, except Share data	3 Months Ended			6 Months Ended				12 Months Ended
	Jun. 30, 2011		Jun. 30, 2010	Jun. 30, 2011		Jun. 30, 2010		Dec. 31, 2010
Common Stock Incentive Plan [Abstract]
Outstanding nonvested shares, beginning of year (in shares)				924,294		853,234		853,234
Shares granted (in shares)				227,014				278,200
Shares vested (in shares)				(244,774)				(206,153)
Shares forfeited (in shares)				(348)				(987)
Outstanding nonvested shares, end of each period (in shares)	906,186			906,186				924,294
Outstanding nonvested shares, beginning of year, Weighted average price (per share)				$ 19.69	[1]	$ 19.14	[1]	$ 19.14	[1]
Shares granted, Weigted average price (per share)				$ 34.00	[1]			$ 28.99	[1]
Shares vested, Weighted average price (per share)				$ 25.26	[1]			$ 23.70	[1]
Shares forfeited, Weigted average price (per share)	$ 31.06	[1]		$ 31.06	[1]			$ 26.03	[1]
Outstanding nonvested shares, end of each period, Weighted average price (per share)	$ 23.79	[1]		$ 23.79	[1]			$ 19.69	[1]
Share-based compensation costs recognized	$ 2.2		$ 1.7	$ 4.3		$ 3.5
Common Stock Award Vesting Period, Maximum (in years)				10 Y
Stock Issued (in shares)	227,014			227,014
Granted shares that vest immediately (in shares)	25,158			25,158
Granted shares that vest in one year (in shares)	5,000			5,000
Granted shares that vest in three years (in shares)	70,400			70,400
Granted shares that vest in five years (in shares)	126,456			126,456
Remaining unamortized share-based compensation expense	$ 21.6			$ 21.6
Stock Options Exercised (in shares)				2,454
Stock Options Exercised, Weighted Average Exercise Price (in dollars per share)	$ 14.70			$ 14.70

[1] Grant date fair value