CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 14,074	$ 17,453	$ 82,091	$ 65,877
Other Comprehensive Income:
Change in unfunded benefit obligation for pension and other postretirement benefit plans - net of taxes of $82, $59, $166 and $113 respectively	308	220	623	425
Total other comprehensive income	308	220	623	425
Comprehensive income	$ 14,382	$ 17,673	$ 82,714	$ 66,302