Common Stock Incentive Plan - Restricted Stock (Details) - Restricted Stock $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2018 director shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 $ / shares shares	Dec. 31, 2016 $ / shares shares
Common Stock Incentive Plan
Remaining unamortized share-based compensation expense | $		$ 12.1
Number of shares
Outstanding nonvested shares, beginning of year		475,768	513,523	456,282
Shares granted		183,952	149,264	260,171
Shares vested		(310,706)	(183,381)	(200,066)
Shares forfeited		(41,193)	(3,638)	(2,864)
Outstanding nonvested shares, end of each period		307,821	475,768	513,523
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share) | $ / shares		$ 52.32	$ 48.33	$ 30.46
Shares granted (in dollars per share) | $ / shares		52.21	59.21	54.14
Shares vested (in dollars per share) | $ / shares		51.05	46.65	43.26
Shares forfeited (in dollars per share) | $ / shares		53.06	56.57	48.15
Outstanding nonvested shares, end of each period (in dollars per share) | $ / shares		$ 53.44	$ 52.32	$ 48.33
Directors
Common Stock Incentive Plan
Vesting period	3 years
Number of directors | director	2
Number of shares
Shares granted	8,000	28,000
Shares vested		(20,000)
Employees
Number of shares
Shares granted		147,952
Three-year vesting period | Directors
Common Stock Incentive Plan
Vesting period		3 years
Number of shares
Shares granted		4,000
Three-year vesting period | Service-based vesting | Directors | Maximum
Common Stock Incentive Plan
Years of service		6 years
Vesting rights percentage		33.33%
Two-year vesting period | Directors
Common Stock Incentive Plan
Vesting period		2 years
Number of shares
Shares granted		4,000
Two-year vesting period | Service-based vesting | Directors
Common Stock Incentive Plan
Years of service		6 years
Vesting rights percentage		50.00%
One-year vesting period | Service-based vesting | Directors
Common Stock Incentive Plan
Years of service		7 years
Vesting rights percentage		100.00%
Immediate vesting | Service-based vesting | Directors | Minimum
Common Stock Incentive Plan
Years of service		8 years
Four year-vesting period | Service-based vesting | Employees
Common Stock Incentive Plan
Vesting period		4 years