Common Stock Incentive Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Plan disclosures
Share-based compensation costs recognized	$ 10,000,000	$ 7,900,000	$ 6,200,000
Additional disclosures
Remaining unamortized share-based compensation expense	17,900,000
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
Number of shares
Outstanding nonvested shares, beginning of year	925,526	924,294	853,234
Shares granted	261,811	247,214	278,200
Shares vested	(290,877)	(245,487)	(206,153)
Shares forfeited	(910)	(495)	(987)
Outstanding nonvested shares, end of each period	895,550	925,526	924,294
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)	$ 20.21	$ 19.69	$ 19.14
Shares granted (in dollars per share)	$ 35.06	$ 33.94	$ 28.99
Shares vested (in dollars per share)	$ 27.47	$ 25.26	$ 23.70
Shares forfeited (in dollars per share)	$ 31.67	$ 31.37	$ 26.03
Outstanding nonvested shares, end of each period (in dollars per share)	$ 19.94	$ 20.21	$ 19.69
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	150,727
Minimum employment requirement	3 years
Incentive Award Plans | Non-executive employees
Plan disclosures
Vesting period	5 years
Additional disclosures
Number of shares of nonvested stock to be received after six full months of employment	200
Incentive Award Plans | Maximum
Plan disclosures
Vesting period	10 years
Stock Option
Number of shares
Outstanding options, beginning of year (in shares)		2,454	5,846
Options exercised (in shares)		(2,454)	(3,392)
Outstanding options, end of year (in shares)			2,454
Exercisable options, end of year (in shares)			2,454
Weighted average exercise price
Outstanding options, beginning of year (in dollars per share)		$ 14.70	$ 14.70
Options exercised (in dollars per share)		$ 14.70	$ 14.70
Outstanding options, end of year (in dollars per share)			$ 14.70
Exercisable options, end of year (in dollars per share)			$ 14.70
Market value of stock (in dollars per share)			$ 34.20
Total intrinsic value of options exercised		48,000	61,000
Aggregate intrinsic value of options outstanding and exercisable			$ 48,000
2012 Plan
Plan disclosures
Authorized shares	3,985,734
Plan term from the date of adoption	10 years