Earnings per Share - Reconciliation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Numerator for basic and diluted EPS — net income allocated to common stockholders	$ 19,628	$ 9,419	$ 23,985	$ 17,022
Denominator for basic EPS — weighted average number of shares (shares)	31,712,959	30,934,832	31,898,939	30,707,895
Add — dilutive effect of:
Weighted average number of incremental shares issuable from restricted stock units (shares)	0	91,000	0	79,000
Denominator for diluted EPS — weighted average number of shares and dilutive potential shares (shares)	31,712,959	31,026,098	31,898,939	30,786,714
Earnings per share:
Basic (usd per share)	$ 0.62	$ 0.30	$ 0.75	$ 0.55
Diluted (usd per share)	$ 0.62	$ 0.30	$ 0.75	$ 0.55
Cogent Partners, LP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from EPS (shares)			334,048