CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income
Beginning balance - Shares at Dec. 31, 2016		6,000,000	49,504,079			417
Beginning balance - Amount at Dec. 31, 2016	$ 2,009,557	$ 150,000	$ 495	$ 955,468	$ 903,187	$ (8)	$ 415
Comprehensive income:
Net income	197,063				197,063
Change in unrealized gain (loss) on available-for-sale securities, net of taxes	13						13
Comprehensive income	197,076
Stock-based compensation expense recognized in earnings	8,079			8,079
Preferred stock dividends	(9,750)				(9,750)
Issuance of stock related to stock-based awards - Shares			106,087
Issuance of stock related to stock-based awards - Amount	(2,241)		$ 1	(2,242)
Ending balance - Amount at Dec. 31, 2017	2,202,721	$ 150,000	$ 496	961,305	1,090,500	$ (8)	428
Ending balance - Shares at Dec. 31, 2017		6,000,000	49,643,761			417
Comprehensive income:
Stock Issued During Period, Shares, Other			33,595
Stock Issued During Period, Value, Other	0		$ 0	0
Net income	300,824				300,824
Change in unrealized gain (loss) on available-for-sale securities, net of taxes	6						6
Comprehensive income	300,830
Stock-based compensation expense recognized in earnings	8,973			8,973
Preferred stock dividends	(9,750)				(9,750)
Issuance of stock related to stock-based awards - Shares			120,242
Issuance of stock related to stock-based awards - Amount	(2,382)		$ 1	(2,383)
Ending balance - Amount at Dec. 31, 2018	2,500,394	$ 150,000	$ 502	967,890	1,381,492	$ (8)	518
Ending balance - Shares at Dec. 31, 2018		6,000,000	50,201,127			417
Comprehensive income:
Stock Issued During Period, Shares, Other			437,124
Stock Issued During Period, Value, Other	0		$ 5	(5)
Net income	322,866				322,866
Change in unrealized gain (loss) on available-for-sale securities, net of taxes	8,432						8,432
Comprehensive income	331,298
Stock-based compensation expense recognized in earnings	11,775			11,775
Preferred stock dividends	(9,750)				(9,750)
Issuance of stock related to stock-based awards - Shares			128,263
Issuance of stock related to stock-based awards - Amount	(1,459)		$ 1	(1,460)
Ending balance - Amount at Dec. 31, 2019	$ 2,832,258	$ 150,000	$ 503	978,205	$ 1,694,608	$ (8)	$ 8,950
Ending balance - Shares at Dec. 31, 2019		6,000,000	50,338,158			417
Comprehensive income:
Stock Issued During Period, Shares, Other			8,768
Stock Issued During Period, Value, Other			$ 0	$ 0