Investments, Debt and Equity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	$ 5,723,434		$ 5,293,918
Unrealized Gains	283,948		103,156
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	966		16,421
Debt Securities, Held-to-maturity, Fair Value	6,006,416		5,380,653
Allowance for credit losses on investment securities held-to-maturity	306	$ 309	0
Net Carrying Value	5,723,128		5,293,918
Interest receivable	56,600		59,000
Held-to-maturity Securities [Member]
Schedule of Investments [Line Items]
Interest receivable	18,500		21,800
Agency CMO [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	116,020		167,443
Unrealized Gains	2,743		1,123
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	261		1,200
Debt Securities, Held-to-maturity, Fair Value	118,502		167,366
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	116,020		167,443
Agency MBS [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	2,595,711		2,957,900
Unrealized Gains	155,221		60,602
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	106		8,733
Debt Securities, Held-to-maturity, Fair Value	2,750,826		3,009,769
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	2,595,711		2,957,900
Agency Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	2,033,826		1,172,491
Unrealized Gains	63,401		6,444
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	599		5,615
Debt Securities, Held-to-maturity, Fair Value	2,096,628		1,173,320
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	2,033,826		1,172,491
Municipal Bonds [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	750,847		740,431
Unrealized Gains	53,457		32,709
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	0		21
Debt Securities, Held-to-maturity, Fair Value	804,304		773,119
Allowance for credit losses on investment securities held-to-maturity	306		0
Net Carrying Value	750,541		740,431
Non-agency CMBS [Member]
Schedule of Investments [Line Items]
Investment securities held-to-maturity, net	227,030		255,653
Unrealized Gains	9,126		2,278
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	0		852
Debt Securities, Held-to-maturity, Fair Value	236,156		257,079
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	$ 227,030		$ 255,653