Common Stock Incentive Plan (Details) (USD $) In Millions, except Share data	3 Months Ended			9 Months Ended				12 Months Ended
	Sep. 30, 2011		Sep. 30, 2010	Sep. 30, 2011		Sep. 30, 2010		Dec. 31, 2010
Common Stock Incentive Plan [Abstract]
Share-based compensation costs recognized	$ 1.8		$ 1.3	$ 6.1		$ 4.8
Common Stock Award Vesting Period, Maximum (in years)				10Y
Equity Instruments Other Than Options, Nonvested [Roll Forward]
Outstanding nonvested shares, beginning of year (in shares)				924,294		853,234		853,234
Shares granted (in shares)				247,014				278,200
Shares vested (in shares)				(245,094)				(206,153)
Shares forfeited (in shares)				(348)				(987)
Outstanding nonvested shares, end of each period (in shares)	925,866			925,866				924,294
Equity Instruments Other than Options, Additional Disclosures [Abstract]
Outstanding nonvested shares, beginning of year, Weighted average price (in dollars per share)				$ 19.69	[1]	$ 19.14	[1]	$ 19.14	[1]
Shares granted, Weighted average price (in dollars per share)				$ 33.94	[1]			$ 28.99	[1]
Shares vested, Weighted average price (in dollars per share)				$ 25.26	[1]			$ 23.70	[1]
Shares forfeited, Weighted average price (in dollars per share)	$ 31.06	[1]		$ 31.06	[1]			$ 26.03	[1]
Outstanding nonvested shares, end of each period, Weighted average price (in dollars per share)	$ 22.12	[1]		$ 22.12	[1]			$ 19.69	[1]
Stock Issued (in shares)	247,014			247,014
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)	146,456			146,456
Remaining unamortized share-based compensation expense	$ 20.5			$ 20.5
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.