Common Stock Incentive Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Plan disclosures
Share-based compensation costs recognized	$ 20.8	$ 10.0	$ 7.9
Additional disclosures
Remaining unamortized share-based compensation expense	$ 16.9
Stock options outstanding	0	0
Performance-based vesting
Plan disclosures
Shares of common stock granted	51,454
Number of shares
Shares of common stock granted	51,454
Non-employee director | Less than six years of service
Additional disclosures
Years of service	6 years
Vesting rights percentage	33.33%
Non-employee director | Six years of service
Additional disclosures
Years of service	6 years
Vesting rights percentage	50.00%
Non-employee director | Seven years of service
Additional disclosures
Years of service	7 years
Vesting rights percentage	100.00%
Employee | Age 55 and below
Additional disclosures
Vesting rights percentage	20.00%
Employee | Age 56
Additional disclosures
Vesting rights percentage	25.00%
Eligible age of employees for vesting of awards	56 years
Employee | Age 57
Additional disclosures
Vesting rights percentage	33.33%
Eligible age of employees for vesting of awards	57 years
Employee | Age 58
Additional disclosures
Vesting rights percentage	50.00%
Eligible age of employees for vesting of awards	58 years
Employee | Age 59
Additional disclosures
Vesting rights percentage	100.00%
Eligible age of employees for vesting of awards	59 years
Employee | Age 60 and above
Additional disclosures
Eligible age of employees for vesting of awards	60 years
Non-executive employees
Additional disclosures
Years of service	6 months
Minimum | Non-employee director | Eight or more years of service
Additional disclosures
Years of service	8 years
Maximum | Employee | Age 55 and below
Additional disclosures
Eligible age of employees for vesting of awards	55 years
Incentive Award Plans
Plan disclosures
Shares of common stock granted	484,060	261,811	247,214
Number of shares
Outstanding nonvested shares, beginning of year	895,550	925,526	924,294
Shares of common stock granted	484,060	261,811	247,214
Shares vested	(654,650)	(290,877)	(245,487)
Shares forfeited	(2,697)	(910)	(495)
Outstanding nonvested shares, end of each period	722,263	895,550	925,526
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)	$ 19.94	$ 20.21	$ 19.69
Shares granted (in dollars per share)	$ 41.13	$ 35.06	$ 33.94
Shares vested (in dollars per share)	$ 30.91	$ 27.47	$ 25.26
Shares forfeited (in dollars per share)	$ 37.30	$ 31.67	$ 31.67
Outstanding nonvested shares, end of each period (in dollars per share)	$ 23.37	$ 19.94	$ 20.21
Additional disclosures
Granted shares that vest over future period	432,606
Granted shares that vest immediately	106,026
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 four year service period	77,180
Granted shares that vest over five year service period	174,411
Incentive Award Plans | Performance-based vesting
Additional disclosures
Granted shares vested in 2013 based on achievement of certain performance metrics	12,864
Granted shares which may vest at the end of 2014, if certain performance metrics are reached	12,864
Granted shares which may vest at the end of 2015, if certain performance metrics are reached	12,864
Granted shares which may vest at the end of 2016, if certain performance metrics are reached	12,864
Incentive Award Plans | Non-executive employees
Additional disclosures
Number of shares of nonvested stock to be received after six full months of employment	200
Vesting period	5 years
Stock Option
Plan disclosures
Shares granted	0
2012 Plan
Plan disclosures
Authorized shares	3,985,734
Plan term from the date of adoption	10 years
2003 Plan
Plan disclosures
Shares of common stock granted	0
Number of shares
Shares of common stock granted	0