Accounting Policies (Details) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 237,832,000		$ 218,760,000		$ 237,832,000	$ 218,760,000	$ 316,473,000	$ 271,719,000
Cash and Cash Equivalents [Line Items]
Net Income (Loss) Attributable to Parent	$ 20,765,000	$ 14,832,000	$ (25,733,000)	$ 8,666,000	$ 35,597,000	$ (17,067,000)
Weighted Average Number of Shares Outstanding, Basic	19,510,000		19,298,000		19,449,000	19,245,000
Weighted Average Number of Shares Outstanding, Diluted	20,115,000		19,298,000		20,197,000	19,245,000
Earnings Per Share, Basic	$ 1.06		$ (1.33)		$ 1.83	$ (0.89)
Earnings Per Share, Diluted	$ 1.03		$ (1.33)		$ 1.76	$ (0.89)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 237,848,000		$ 218,760,000		$ 237,848,000	$ 218,760,000	316,489,000	271,719,000
Restricted Stock Units (RSUs)
Cash and Cash Equivalents [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	498,000		0		593,000	0
Performance Shares
Cash and Cash Equivalents [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	107,000		0		155,000	0
Other Noncurrent Assets [Member]
Accounting Policies [Abstract]
Restricted Cash	$ 16,000		$ 0		$ 16,000	$ 0	16,000	0
Cash and Cash Equivalents [Line Items]
Restricted Cash	$ 16,000		$ 0		$ 16,000	$ 0	$ 16,000	$ 0