Common Stock Repurchased Shares (Details) - USD ($)	1 Months Ended	3 Months Ended			6 Months Ended
	Jan. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Dec. 31, 2017
Numbers of Shares Repurchased
Cumulative repurchases as of December 31, 2017 (in shares)	1,004,793				1,004,793
Six months ended June 30, 2018 (in shares)	10,183	82,256	578	276,624	109,314	99,131
Cumulative repurchases as of June 30, 2018 (in shares)					1,114,107	1,004,793
Average Price per Share
Cumulative repurchases as of December 31, 2017 (in dollars per share)	$ 22.48				$ 22.48
Six months ended June 30, 2018 (in dollars per share)	$ 20.26	$ 21.25	$ 23.68	$ 20.15	20.26	$ 20.26
Cumulative repurchases as of June 30, 2018 (in dollars per share)					$ 22.26	$ 22.48
Common stock repurchases	$ 200,000	$ 1,700,000	$ 13,700	$ 5,600,000	$ 4,600,000	$ 2,000,000
Share Repurchase Program, Shares Repurchased Following Death or Qualifying Disability
Average Price per Share
Percentage of stock repurchase requests approved during the period		100.00%