Common Stock Incentive Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Dec. 31, 2011
Common Stock Incentive Plan [Abstract]
Share-based compensation costs recognized	$ 3.0		$ 2.2
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)	925,526		924,294		924,294
Shares granted (in shares)	235,811				247,214
Shares vested (in shares)	(250,876)				(245,487)
Shares forfeited (in shares)	(275)				(495)
Outstanding nonvested shares, end of each period (in shares)	910,186				925,526
Equity Instruments Other than Options, Additional Disclosures [Abstract]
Outstanding nonvested shares, beginning of year, Weighted average price (in dollars per share)	$ 20.21	[1]	$ 19.69	[1]	$ 19.69	[1]
Shares granted, Weighted average price (in dollars per share)	$ 34.57	[1]			$ 33.94	[1]
Shares vested, Weighted average price (in dollars per share)	$ 26.51	[1]			$ 25.26	[1]
Shares forfeited, Weighted average price (in dollars per share)	$ 26.02	[1]			$ 31.37	[1]
Outstanding nonvested shares, end of each period, Weighted average price (in dollars per share)	$ 26.25	[1]			$ 20.21	[1]
Stock Issued (in shares)	235,811
Granted shares that vest immediately (in shares)	18,484
Granted shares that vest in two years (in shares)	64,600
Granted shares that vest in three years (in shares)	4,000
Granted shares that vest in five years (in shares)	148,727
Remaining unamortized share-based compensation expense	$ 23.9

[1] Grant date fair value.