Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Stockholders' equity at beginning of period at Dec. 31, 2018	$ 2,399,101	$ 0	$ 90,369	$ 811,186	$ (52,432)	$ 1,549,978
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	25,940					25,940
Other comprehensive income	1,691,861				1,691,861
Share-based compensation	9,402			9,402
Issuance of common stock under compensation plans	412		638	(226)
Stockholders' equity at end of period at Sep. 30, 2019	4,126,716	0	91,007	820,362	1,639,429	1,575,918
Stockholders' equity at beginning of period at Jun. 30, 2019	3,497,475	0	90,936	817,997	1,049,984	1,538,558
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37,360					37,360
Other comprehensive income	589,445				589,445
Share-based compensation	2,042			2,042
Issuance of common stock under compensation plans	394		71	323
Stockholders' equity at end of period at Sep. 30, 2019	4,126,716	0	91,007	820,362	1,639,429	1,575,918
Stockholders' equity at beginning of period at Dec. 31, 2019	4,570,119	16	91,107	1,212,311	1,497,921	1,768,764
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	662,718					662,718
Other comprehensive income	614,190				614,190
Issuance of preferred stock	290,260	12		290,248
Share-based compensation	7,515			7,515
Issuance of common stock under compensation plans	1,738		825	913
Cumulative effect of change in accounting principle	(9,295)					(9,295)
Dividends on preferred stock	(18,511)					(18,511)
Stockholders' equity at end of period at Sep. 30, 2020	6,118,734	28	91,932	1,510,987	2,112,111	2,403,676
Stockholders' equity at beginning of period at Jun. 30, 2020	5,062,059	28	91,595	1,508,171	1,719,839	1,742,426
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	667,200					667,200
Other comprehensive income	392,272				392,272
Share-based compensation	3,121			3,121
Issuance of common stock under compensation plans	32		337	(305)
Dividends on preferred stock	(5,950)					(5,950)
Stockholders' equity at end of period at Sep. 30, 2020	$ 6,118,734	$ 28	$ 91,932	$ 1,510,987	$ 2,112,111	$ 2,403,676