Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Cumulative Earnings	Cumulative Distributions to Stockholders
Beginning balance (in shares) at Dec. 31, 2017		76,599,972
Balance at beginning of period at Dec. 31, 2017	$ 1,212,287	$ 766	$ 1,673,845	$ 85,248	$ 1,290,341	$ (1,837,913)
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 (in shares)		8,738,319
Issuance of common stock	142,228	$ 88	142,140
Direct stock purchase and dividend reinvestment plan (in shares)		113,004
Direct stock purchase and dividend reinvestment plan	1,706	$ 1	1,705
Employee stock purchase and incentive plans (in shares)		473,878
Employee stock purchase and incentive plans	(4,466)	$ 4	(4,470)
Non-cash equity award compensation	13,736		13,736
Share repurchases (in shares)		(1,040,829)
Share repurchases	(15,544)	$ (10)	(15,534)
Common dividends declared	(96,802)					(96,802)
Ending balance (in shares) at Dec. 31, 2018		84,884,344
Balance at End of Period at Dec. 31, 2018	1,348,794	$ 849	1,811,422	61,297	1,409,941	(1,934,715)
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 (in shares)		28,724,645
Issuance of common stock	442,172	$ 288	441,884
Direct stock purchase and dividend reinvestment plan (in shares)		399,838
Direct stock purchase and dividend reinvestment plan	6,307	$ 4	6,303
Employee stock purchase and incentive plans (in shares)		344,209
Employee stock purchase and incentive plans	(4,946)	$ 3	(4,949)
Non-cash equity award compensation	14,957		14,957
Common dividends declared	(129,452)					(129,452)
Ending balance (in shares) at Dec. 31, 2019		114,353,036
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	(581,847)				(581,847)
Other comprehensive income (loss)	(45,734)			(45,734)
Issuance of common stock (in shares)		350,088
Issuance of common stock	5,547	$ 3	5,544
Direct stock purchase and dividend reinvestment plan (in shares)		0
Employee stock purchase and incentive plans (in shares)		434,217
Employee stock purchase and incentive plans	(3,952)	$ 4	(3,956)
Non-cash equity award compensation	15,298		15,298
Share repurchases (in shares)		(3,047,335)
Share repurchases	(21,659)	$ (30)	(21,629)
Common dividends declared	(83,985)					(83,985)
Ending balance (in shares) at Dec. 31, 2020		112,090,006
Balance at End of Period at Dec. 31, 2020	$ 1,110,899	$ 1,121	$ 2,264,874	$ (4,221)	$ 997,277	$ (2,148,152)