Common Stock - Stock Repurchase Program (Details) - USD ($)	3 Months Ended		9 Months Ended		40 Months Ended	71 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018	Jun. 30, 2015	Nov. 30, 2012
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, remaining amount	$ 49,700,000		$ 49,700,000		$ 49,700,000	$ 49,700,000
Share Repurchase Program, November 2012
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount (up to)								$ 25,000,000
Repurchase of common stock						$ 25,000,000
Share Repurchase Program, June 2015
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount (up to)							$ 50,000,000
Repurchase of common stock		$ 3,700,000		$ 4,200,000	$ 300,000
Repurchase of common stock during period (in shares)	0	630,000	0	680,000
Average cost per share (in dollars per share)		$ 5.85		$ 6.14