Investments, Debt and Equity Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	$ 5,477,126		$ 5,293,918
Unrealized Gains	268,729		103,156
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	296		16,421
Debt Securities, Held-to-maturity, Fair Value	5,745,559		5,380,653
Allowance for credit losses on investment securities held-to-maturity	309	$ 312	0
Net Carrying Value	5,476,817		5,293,918
Interest receivable	55,500		59,000
Held-to-maturity Securities [Member]
Schedule of Investments [Line Items]
Interest receivable	21,800		21,800
Agency CMO [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	138,529		167,443
Unrealized Gains	3,469		1,123
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	223		1,200
Debt Securities, Held-to-maturity, Fair Value	141,775		167,366
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	138,529		167,443
Agency MBS [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	2,766,764		2,957,900
Unrealized Gains	148,003		60,602
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	73		8,733
Debt Securities, Held-to-maturity, Fair Value	2,914,694		3,009,769
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	2,766,764		2,957,900
Agency Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	1,585,929		1,172,491
Unrealized Gains	60,543		6,444
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	0		5,615
Debt Securities, Held-to-maturity, Fair Value	1,646,472		1,173,320
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	1,585,929		1,172,491
Municipal Bonds [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	752,537		740,431
Unrealized Gains	49,298		32,709
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	0		21
Debt Securities, Held-to-maturity, Fair Value	801,835		773,119
Allowance for credit losses on investment securities held-to-maturity	309		0
Net Carrying Value	752,228		740,431
Non-agency CMBS [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	233,367		255,653
Unrealized Gains	7,416		2,278
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	0		852
Debt Securities, Held-to-maturity, Fair Value	240,783		257,079
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	$ 233,367		$ 255,653