Common Stock Repurchased Shares (Details) - USD ($)	1 Months Ended	3 Months Ended			9 Months Ended	12 Months Ended
	Jul. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Numbers of Shares Repurchased
Cumulative repurchased shares, beginning balance (in shares)				645,335	645,335
Shares repurchased during period (in shares)		82,256	578	276,624	359,458
Cumulative repurchased shares, ending balance (in shares)		1,004,793			1,004,793	645,335
Weighted-Average Price per Share
Weighted average price per share, cumulative shares repurchased (in dollars per share)		$ 22.48			$ 22.48	$ 23.63
Weighted average price per share, shares repurchased during period (in dollars per share)		$ 21.25	$ 23.68	$ 20.15	$ 20.41
Stock repurchased during period, value		$ 1,700,000	$ 13,700	$ 5,600,000	$ 7,337,000
Rejected shares (in shares)						902,420
Rejected shares						$ 18,100,000
Weighted average price per share, rejected shares (in dollars per share)						$ 20.03
Shares Repurchased Following Death or Qualifying Disability
Weighted-Average Price per Share
Percentage of repurchase requests approved	100.00%