Incentive and Deferred Compensation Plans (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred executive and outside directors' compensation liability
Total plan awards and earnings	$ 19,713	$ 18,945	$ 23,327
Total plan awards paid	(20,621)	(20,418)	(14,317)
Incentive plan and deferred compensation liabilities at end of period	30,057	32,908	37,457
Annual incentive plan (AIP)
Deferred executive and outside directors' compensation liability
Annual incentive plan awards	6,118	6,460	7,057
Long-term incentive plan (LTIP)
Deferred executive and outside directors' compensation liability
Long-term incentive plan awards	10,931	11,321	14,228
Forfeitures	(593)	(3,117)	(821)
Deferred compensation plans and stock compensation plan for outside directors
Deferred executive and outside directors' compensation liability
Employer match and hypothetical earnings on deferred compensation	2,664	1,164	2,042
Compensation deferred	680	1,214	996
Distributions from the deferred compensation plans	(853)	(435)	(1,688)
Funding of rabbi trust	$ (1,177)	$ (738)	$ (9,018)