Stock-Based Compensation Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 Plan	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock-based compensation plans	3
Stock-based compensation expense	$ 4,843,000	$ 3,668,000	$ 2,358,000
Restricted and performance-based stock, weighted average grant date fair value per share [Roll Forward]
Weighted-average grant date fair value	18,400,000	12,300,000	7,000,000
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance (in shares)	9,875
Options, shares [Roll Forward]
Outstanding, beginning of period (in shares)	16,875	29,150
Expired (in shares)	0	0
Exercised (in shares)	(7,000)	(12,275)
Forfeited, other (in shares)	0	0
Outstanding, end of period (in shares)	9,875	16,875	29,150
Options exercisable, end of period (in shares)	9,875
Options, weighted average exercise price [Roll Forward]
Outstanding, beginning of period (in dollars per share)	$ 12.14	$ 12.20
Expired (in dollars per share)	$ 0	$ 0
Exercised (in dollars per share)	$ 13.76	$ 12.29
Forfeited, other (in dollars per share)	$ 0	$ 0
Outstanding, end of period (in dollars per share)	$ 10.99	$ 12.14	$ 12.20
Options exercisable, end of period (in dollars per share)	$ 10.99
Options, additional disclosures [Abstract]
Outstanding, weighted average remaining contractual term, beginning of period	0 years 4 months 24 days	1 year	1 year 10 months 24 days
Outstanding, weighted average remaining contractual term, end of period	0 years 4 months 24 days	1 year	1 year 10 months 24 days
Options exercisable, weighted average remaining contractual term, end of period	0 years 4 months 24 days
Aggregate intrinsic value of outstanding stock options	300,000
Total intrinsic value of options exercised	200,000	200,000
Options granted (in shares)	0
Performance-Based Shares [Member]
Restricted and Performance Stock Grants [Abstract]
Measuring period for performance-based shares	3 years
Award vesting rights	Selected executives and other key personnel are granted performance awards whose vesting is contingent upon meeting various performance measures with a retention feature. This component of compensation is designed to encourage the long-term retention of key executives and to tie executive compensation directly to Company performance and the long-term enhancement of shareholder value. Performance-based shares are subject to a three year measuring period and the achievement of performance targets and, depending upon the achievement of such performance targets, they may become vested on the third anniversary of the date of grant.
Restricted Shares [Member]
Restricted and Performance Stock Grants [Abstract]
Award vesting rights	Restricted shares granted to employees become fully vested upon the third anniversary of the date of grant; and for selected key executives certain additional restricted share grants vest 25% upon the attainment of age 60, 25% upon the attainment of age 63 and become fully vested upon the attainment of age 65. Restricted shares granted to directors become fully vested upon the first anniversary of the date of grant.
Restricted and Performance-Based Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	4,300,000	3,200,000	1,900,000
Stock-based compensation expense, net of tax	2,800,000	2,000,000	1,200,000
Options, additional disclosures [Abstract]
Unamortized compensation expense	$ 11,800,000	$ 9,500,000
Restricted and performance-based stock, shares [Roll Forward]
Beginning of period (in shares)	630,600	533,625
Granted (in shares)	256,443	215,975
Vested (in shares)	(132,325)	(108,875)
Forfeited (in shares)	(5,700)	(10,125)
End of period (in shares)	749,018	630,600	533,625
Restricted and performance-based stock, weighted average grant date fair value per share [Roll Forward]
Beginning of period (in dollars per share)	$ 19.47	$ 13.03
Granted (in dollars per share)	$ 32.22	$ 30.53
Vested (in dollars per share)	$ 15.50	$ 10.33
Forfeited (in dollars per share)	$ 18.77	$ 14.49
End of period (in dollars per share)	$ 24.62	$ 19.47	$ 13.03
Restricted and Performance-Based Shares [Member] | Employees [Member]
Options, additional disclosures [Abstract]
Weighted average period of recognition for unrecognized compensation expense	5 years 1 month 6 days	5 years 7 months 6 days
Restricted and Performance Stock Grants [Abstract]
Estimated forfeitures (in hundredths)	5.00%
Restricted and Performance-Based Shares [Member] | Executives [Member]
Restricted and Performance Stock Grants [Abstract]
Estimated forfeitures (in hundredths)	0.00%
Restricted and Performance-Based Shares [Member] | Directors [Member]
Options, additional disclosures [Abstract]
Weighted average period of recognition for unrecognized compensation expense	0 years 3 months 18 days	0 years 3 months 18 days
Restricted and Performance Stock Grants [Abstract]
Estimated forfeitures (in hundredths)	0.00%
Restricted and Performance-Based Shares [Member] | Age 63 [Member]
Restricted and Performance Stock Grants [Abstract]
Vesting percentage (in hundredths)	25.00%
Restricted and Performance-Based Shares [Member] | Age 65 [Member]
Restricted and Performance Stock Grants [Abstract]
Vesting percentage (in hundredths)	25.00%
2006 Omnibus Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance (in shares)	1,900,000
Share-based compensation basic and diluted (in dollars per share)	$ 0.12	$ 0.09	$ 0.05
2006 Omnibus Incentive Plan [Member] | Stock Options [Member]
Options, shares [Roll Forward]
Outstanding, end of period (in shares)	0
2006 Omnibus Incentive Plan [Member] | Performance-Based Shares [Member]
Restricted and performance-based stock, shares [Roll Forward]
Granted (in shares)	46,250	43,000
2006 Omnibus Incentive Plan [Member] | Restricted Shares [Member]
Restricted and performance-based stock, shares [Roll Forward]
Granted (in shares)	210,193	172,975
2006 Omnibus Incentive Plan [Member] | Restricted and Performance-Based Shares [Member]
Restricted and Performance Stock Grants [Abstract]
Restricted and performance-based stock grants issued, net of forfeitures (in shares)	1,388,802
Common stock available for future grants (in shares)	511,198
2004 Omnibus Incentive Plan [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance (in shares)	500,000
Stock option expiration period	10 years
Options, shares [Roll Forward]
Outstanding, end of period (in shares)	7,875
2004 Omnibus Incentive Plan [Member] | Stock Options [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation option exercisable period	3 years
2004 Omnibus Incentive Plan [Member] | Stock Options [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation option exercisable period	5 years
2004 Independent Directors' Stock Option Plan [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance (in shares)	50,000
Stock option expiration period	10 years
Share based compensation option exercisable period	1 year
Options, shares [Roll Forward]
Outstanding, end of period (in shares)	2,000