Stock-Based Compensation (Details) - USD ($)	6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Mar. 31, 2015	Feb. 28, 2015	Jan. 01, 2015	Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense related to stock awards	$ 7,200,000		$ 5,700,000
Stock Options, Shares
Outstanding at beginning of period (in shares)	569,342
Granted (in shares)	0
Exercised (in shares)	(236,635)
Forfeited (in shares)	(2,000)
Expired (in shares)	(8,200)
Outstanding at end of period (in shares)	322,507		569,342
Stock Options, Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 19.35
Granted (in dollars per share)	0.00
Exercised (in dollars per share)	27.76
Forfeited (in dollars per share)	12.22
Expired (in dollars per share)	24.85
Outstanding at end of period (in dollars per share)	13.08		$ 19.35
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average grant date fair value of options granted (in dollars per share)	$ 0.00	$ 0.00
Total fair value of options that vested	$ 0	$ 0
Total intrinsic value of options exercised	$ 1,145,000	$ 1,759,000
Chairman, President, and Chief Executive Officer [Member] | Common Stock [Member] | Achieves a specified improvement in total shareholder return at the end of 2016 compared to 2011 and she remains an employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares received if achieves specified improvement in total shareholders return (in shares)							135,000
Chairman, President, and Chief Executive Officer [Member] | Common Stock [Member] | Achieves a specified improvement in total shareholder return at the end of 2014 compared to 2011 and she remains an employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares received if achieves specified improvement in total shareholders return (in shares)							35,000
Share-based Compensation Arrangement by share-based payment award, Number of shares approved by the board					35,000
Chairman, President, and Chief Executive Officer [Member] | Common Stock [Member] | Achieves a specific performance target by the end of 2019 and she remains an employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares received if achieves specified improvement in total shareholders return (in shares)				53,859
Chairman, President, and Chief Executive Officer [Member] | Common Stock [Member] | Achieves a specific performance target by the end of 2017 and she remains an employee [Member] [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares received if achieves specified improvement in total shareholders return (in shares)				17,953
Executive Vice President and Chief Financial Officer [Member] | Common Stock [Member] | Achieved performance target for 2015 and 2016 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, purchase price of common stock						$ 100,000
Executive Vice President and Chief Financial Officer [Member] | Common Stock [Member] | Achieved performance target for 2015, 2016 and 2017 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, purchase price of common stock						$ 275,000
Restricted Stock and Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period of time stock expense is expected to be recognized	1 year 8 months 12 days		1 year 4 months 24 days
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected quarterly dividends per share (in dollars per share)	$ 0.22	$ 0.20
Risk-free interest rate	0.94%	0.92%
Restricted Stock, Shares
Outstanding at beginning of period (in shares)	245,094
Granted (in shares)	190,276
Exercised (in shares)	(203,423)
Forfeited (in shares)	0
Outstanding at end of period (in shares)	231,947		245,094
Restricted Stock, Weighted- Average Grant Date Fair Value
Outstanding at beginning of period (in dollars per share)	$ 24.81
Granted (in dollars per share)	26.49	$ 20.34
Exercised (in dollars per share)	23.44
Forfeited (in dollars per share)	0.00
Outstanding at end of period (in dollars per share)	27.40		$ 24.81
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average grant date fair value (in dollars per share)	$ 26.49	$ 20.34
Total fair value of restricted shares that vested	$ 4,768,000	$ 6,470,000
Performance Shares [Member] | Executive [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	79,619
Shares exercised (in shares)	74,697
Share-based compensation, weighted percentage assigned to achieving market targets	60.00%
Share-based compensation, weighted percentage assigned to achieving performance targets	40.00%
Share-based compensation, maximum number of shares awarded in year 1 (in shares)	165,628
Share-based compensation, maximum number of shares awarded in year 2 (in shares)	166,797
Share-based compensation, maximum number of shares awarded in year 3 (in shares)	147,031
Performance period	3 years
Market-Based Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate	0.97%	1.00%
Dividend yield	2.74%	2.87%
Expected volatility	20.44%	18.73%
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of stock options outstanding	$ 7,200,000
Weighted-average remaining contract life	2 years 5 months 12 days
Number of outstanding stock options with an exercise price greater than the closing price (in shares)	0
Performance Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Vesting rate	100.00%