Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Cumulative Earnings	Cumulative Distributions to Stockholders
Balance at beginning of period at Dec. 31, 2016	$ 1,149,428	$ 768	$ 1,676,486	$ 71,853	$ 1,149,935	$ (1,749,614)
Beginning balance (in shares) at Dec. 31, 2016		76,834,663
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	140,406				140,406
Other comprehensive income (loss)	13,395			13,395
Employee stock purchase and incentive plans	(3,834)	$ 4	(3,838)
Employee stock purchase and incentive plans (in shares)		375,651
Non-cash equity award compensation	10,378		10,378
Share repurchases	(9,187)	$ (6)	(9,181)
Share repurchases (in shares)		(610,342)
Common dividends declared	(88,299)					(88,299)
Balance at End of Period at Dec. 31, 2017	1,212,287	$ 766	1,673,845	85,248	1,290,341	(1,837,913)
Ending balance (in shares) at Dec. 31, 2017		76,599,972
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	119,600				119,600
Other comprehensive income (loss)	(23,951)			(23,951)
Issuance of common stock	$ 142,228	$ 88	142,140
Issuance of common stock (in shares)	7,187,500	8,738,319
Direct stock purchase and dividend reinvestment plan	$ 1,706	$ 1	1,705
Direct stock purchase and dividend reinvestment plan (in shares)		113,004
Employee stock purchase and incentive plans	(4,466)	$ 4	(4,470)
Employee stock purchase and incentive plans (in shares)		473,878
Non-cash equity award compensation	13,736		13,736
Share repurchases	(15,544)	$ (10)	(15,534)
Share repurchases (in shares)		(1,040,829)
Common dividends declared	(96,802)					(96,802)
Balance at End of Period at Dec. 31, 2018	1,348,794	$ 849	1,811,422	61,297	1,409,941	(1,934,715)
Ending balance (in shares) at Dec. 31, 2018		84,884,344
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	169,183				169,183
Other comprehensive income (loss)	(19,784)			(19,784)
Issuance of common stock	442,172	$ 288	441,884
Issuance of common stock (in shares)		28,724,645
Direct stock purchase and dividend reinvestment plan	6,307	$ 4	6,303
Direct stock purchase and dividend reinvestment plan (in shares)		399,838
Employee stock purchase and incentive plans	(4,946)	$ 3	(4,949)
Employee stock purchase and incentive plans (in shares)		344,209
Non-cash equity award compensation	14,957		14,957
Common dividends declared	(129,452)					(129,452)
Balance at End of Period at Dec. 31, 2019	$ 1,827,231	$ 1,144	$ 2,269,617	$ 41,513	$ 1,579,124	$ (2,064,167)
Ending balance (in shares) at Dec. 31, 2019		114,353,036