Common Stock Incentive Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended			12 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Incentive Award Plans	Dec. 31, 2012 Incentive Award Plans	Jun. 30, 2013 2012 Plan	Jun. 30, 2013 2012 Plan Maximum
Plan disclosures
Authorized shares							3,985,734
Plan term from the date of adoption							10 years
Share-based compensation costs recognized	$ 3.7	$ 2.6	$ 7.5	$ 5.6
Vesting period					10 years			10 years
Number of shares
Outstanding nonvested shares, beginning of year					895,550	925,526
Shares granted					272,787	261,811
Shares vested					(324,625)	(290,877)
Shares forfeited					(2,143)	(910)
Outstanding nonvested shares, end of each period					841,569	895,550
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)					$ 19.94	$ 20.21
Shares granted (in dollars per share)					$ 42.89	$ 35.06
Shares vested (in dollars per share)					$ 32.14	$ 27.47
Shares forfeited (in dollars per share)					$ 35.84	$ 31.67
Outstanding nonvested shares, end of each period (in dollars per share)					$ 26.12	$ 19.94
Additional disclosures
Granted shares that vest immediately					25,662
Granted Shares that Vest over one Year Service Period					62,989
Granted shares that vest over three year service period					12,000
Granted shares that vest over five year service period					172,136
Remaining unamortized share-based compensation expense	$ 22.0		$ 22.0