Earnings Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended								12 Months Ended
		Sep. 11, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 10, 2018
Numerator for Earnings per Common Share-Basic:
Net Income											$ 105,334	$ 72,968	$ 71,392
Less: Income allocated to participating shares											304	242	225
Net Income Allocated to Common Shareholders			$ 26,854	$ 30,881	$ 21,436	$ 26,163	$ 9,294	$ 22,721	$ 22,765	$ 18,188	105,030	72,726	71,167
Numerator for Earnings per Common Share—Diluted:
Net Income											$ 105,334	$ 72,968	$ 71,392
Denominators:
Weighted Average Common Shares Outstanding-Basic (in shares)											34,775,784	34,729,376	34,677,738
Add: Dilutive potential common shares (in shares)											199,625	225,391	95,432
Denominator for Treasury Stock Method-Diluted (in shares)											34,975,409	34,954,767	34,773,170
Weighted Average Common Shares Outstanding-Basic (in shares)											34,775,784	34,729,376	34,677,738
Add: Average participating shares outstanding (in shares)											100,733	115,418	109,755
Denominator for Two-Class Method-Diluted (in shares)											34,876,517	34,844,794	34,787,493
Earnings per common share—basic (in USD per share)											$ 3.03	$ 2.10	$ 2.06
Earnings per common share—diluted (in USD per share)			$ 0.77	$ 0.88	$ 0.61	$ 0.75	$ 0.27	$ 0.65	$ 0.65	$ 0.52	$ 3.01	$ 2.09	$ 2.05
Repurchase of warrant		$ 7,700									$ 7,652	$ 0	$ 0
Number of shares of common stock that holder of warrants have right to purchase (in shares)														517,012
Warrants exercise price (in USD per share)			$ 31.53				$ 31.53				$ 31.53	$ 31.53	$ 31.53	$ 31.53
Warrants
Denominators:
Anti-dilutive securities excluded from dilutive potential common shares	[1]										267,106	438,681	517,012
Restricted Stock
Denominators:
Anti-dilutive securities excluded from dilutive potential common shares											81,587	88,578	116,749

[1] We repurchased our outstanding warrant on September 11, 2018 for $7.7 million. Prior to the repurchase, the warrant provided the holder the right to 517,012 shares of common stock at a strike price of $31.53 per share via cashless exercise.