Employee Benefit and Deferred Compensation Plans - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) point shares	Jun. 30, 2017 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum retirement age for benefits			55 years
Maximum retirement age for benefits			65 years
Eligible employee years of service			15 years
Number of points for eligibility | point			70
Minimum eligible age for medicare coverage			65 years
Life insurance coverage face value | $			$ 5,000
Age limit for life insurance coverage provided by the Company			70 years
Age at which retiree's pay for life insurance coverage at their sole expense			70 years
Option expiration period			10 years
Award vesting period			3 years
Stock options granted (in shares) | shares	0		0	0
Treasury shares reissued (in shares) | shares			93,511
Total stock-based compensation expense | $	$ 1,920,000	$ 1,237,000	$ 3,712,000	$ 2,411,000
Performance Based Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allowable variance			5.00%