Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Shares	Additional Paid-In Capital	Cumulative Dividends in Excess of Net Income	Accumulated Other Comprehensive Income
Balance, beginning at Dec. 31, 2017	$ 1,048,428	$ 599	$ 1,190,250	$ (144,734)	$ 2,313
Beginning balance, common shares, shares outstanding at Dec. 31, 2017		59,941,088
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of common shares (in shares)		(42,137)
Repurchase of common shares, value	(1,146)		(1,146)
Issuance of restricted common shares (in shares)		481,449
Issuance of restricted common shares, value	0	$ 5	(5)
Issuance of unrestricted common shares (in shares)		764
Issuance of unrestricted common shares, value	21		21
Amortization of deferred compensation	1,927		1,927
Declaration of dividends on common shares	(24,152)			(24,152)
Net income	6,549			6,549
Other comprehensive income (loss)	3,482				3,482
Balance, ending at Mar. 31, 2018	1,035,109	$ 604	1,191,047	(162,337)	5,795
Ending balance, common shares, shares outstanding at Mar. 31, 2018		60,381,164
Balance, beginning at Dec. 31, 2017	1,048,428	$ 599	1,190,250	(144,734)	2,313
Beginning balance, common shares, shares outstanding at Dec. 31, 2017		59,941,088
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of common shares, value	(1,146)
Net income	30,359
Balance, ending at Jun. 30, 2018	1,037,856	$ 604	1,192,883	(162,685)	7,054
Ending balance, common shares, shares outstanding at Jun. 30, 2018		60,396,063
Balance, beginning at Mar. 31, 2018	1,035,109	$ 604	1,191,047	(162,337)	5,795
Beginning balance, common shares, shares outstanding at Mar. 31, 2018		60,381,164
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted common shares (in shares)		14,130
Issuance of restricted common shares, value	0
Issuance of unrestricted common shares (in shares)		769
Issuance of unrestricted common shares, value	24		24
Amortization of deferred compensation	1,812		1,812
Declaration of dividends on common shares	(24,158)			(24,158)
Net income	23,810			23,810
Other comprehensive income (loss)	1,259				1,259
Balance, ending at Jun. 30, 2018	1,037,856	$ 604	1,192,883	(162,685)	7,054
Ending balance, common shares, shares outstanding at Jun. 30, 2018		60,396,063
Balance, beginning at Dec. 31, 2018	$ 1,053,978	$ 603	1,193,455	(144,341)	4,261
Beginning balance, common shares, shares outstanding at Dec. 31, 2018	60,263,670	60,263,670
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of common shares (in shares)		(47,653)
Repurchase of common shares, value	$ (1,163)		(1,163)
Issuance of restricted common shares (in shares)		548,419
Issuance of restricted common shares, value	0	$ 5	(5)
Issuance of unrestricted common shares (in shares)		1,360
Issuance of unrestricted common shares, value	38		38
Amortization of deferred compensation	1,895		1,895
Declaration of dividends on common shares	(24,306)			(24,306)
Net income	8,251			8,251
Other comprehensive income (loss)	(2,115)				(2,115)
Balance, ending at Mar. 31, 2019	1,036,578	$ 608	1,194,220	(160,396)	2,146
Ending balance, common shares, shares outstanding at Mar. 31, 2019		60,765,796
Balance, beginning at Dec. 31, 2018	$ 1,053,978	$ 603	1,193,455	(144,341)	4,261
Beginning balance, common shares, shares outstanding at Dec. 31, 2018	60,263,670	60,263,670
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of common shares (in shares)	(47,653)
Repurchase of common shares, value	$ (1,163)
Issuance of restricted common shares (in shares)	548,419
Issuance of unrestricted common shares (in shares)	1,360
Net income	$ 26,492
Balance, ending at Jun. 30, 2019	$ 1,028,791	$ 608	1,196,084	(166,460)	(1,441)
Ending balance, common shares, shares outstanding at Jun. 30, 2019	60,765,796	60,765,796
Balance, beginning at Mar. 31, 2019	$ 1,036,578	$ 608	1,194,220	(160,396)	2,146
Beginning balance, common shares, shares outstanding at Mar. 31, 2019		60,765,796
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amortization of deferred compensation	1,864		1,864
Declaration of dividends on common shares	(24,305)			(24,305)
Net income	18,241			18,241
Other comprehensive income (loss)	(3,587)				(3,587)
Balance, ending at Jun. 30, 2019	$ 1,028,791	$ 608	$ 1,196,084	$ (166,460)	$ (1,441)
Ending balance, common shares, shares outstanding at Jun. 30, 2019	60,765,796	60,765,796