Common Stock Incentive Plans (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended			12 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Restricted Stock	Dec. 31, 2011 Restricted Stock	Jun. 30, 2012 Restricted Stock Maximum	Jun. 30, 2012 2012 Plan
Plan disclosures
Authorized shares								3,985,734
Limit on awards to individual employee in any calendar year (in shares)								3,200,000
Share-based compensation costs recognized	$ 2.6	$ 2.2	$ 5.6	$ 4.3
Vesting period							10 years
Number of shares
Outstanding nonvested shares, beginning of year					925,526	924,294
Shares granted					260,411	247,214
Shares vested					(286,276)	(245,487)
Shares forfeited					(275)	(495)
Outstanding nonvested shares, end of each period					899,386	925,526
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)					$ 20.21	$ 19.69
Shares granted (in dollars per share)					$ 35.03	$ 33.94
Shares vested (in dollars per share)					$ 27.39	$ 25.26
Shares forfeited (in dollars per share)					$ 26.02	$ 31.37
Outstanding nonvested shares, end of each period (in dollars per share)					$ 24.76	$ 20.21
Additional disclosures
Granted shares that vest immediately					26,484
Granted shares that vest over two year service period					68,600
Granted shares that vest over three year service period					16,000
Granted shares that vest over five year service period					149,327
Remaining unamortized share-based compensation expense	$ 22.3		$ 22.3