Stockholders' Equity (Details) - USD ($)	3 Months Ended					9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Nov. 30, 2018	Dec. 31, 2017
Stockholders' equity
Common stock, outstanding shares (in shares)	16,815,000					16,815,000		16,859,000
Shares purchased (Note 9)	$ 2,900,000	$ 1,600,000	$ 300,000	$ 1,000,000.0	$ 5,300,000
Stock acquired (in shares)						2,432
Maximum
Stockholders' equity
Stock acquired						$ 200,000
2019 Stock Purchase Plan
Stockholders' equity
Common stock purchased under the stock purchase plan (in shares)						79,676	0
Shares purchased (Note 9)						$ 4,900,000	$ 0
2019 Stock Purchase Plan | Maximum
Stockholders' equity
Authorized amount of repurchase under the stock purchase plan (up to)									$ 25,000,000
2018 Stock Purchase Plan
Stockholders' equity
Common stock purchased under the stock purchase plan (in shares)						0	79,179
Shares purchased (Note 9)						$ 0	$ 6,300,000
2018 Stock Purchase Plan | Maximum
Stockholders' equity
Authorized amount of repurchase under the stock purchase plan (up to)								$ 25,000,000
2017 Stock Purchase Plan | Maximum
Stockholders' equity
Authorized amount of repurchase under the stock purchase plan (up to)										$ 25,000,000