Investments, Debt and Equity Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Amortized Cost(1)	$ 5,623,625		$ 5,568,188
Unrealized Gains	190,822		269,717
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(20,292)		(2,541)
Debt Securities, Held-to-maturity, Fair Value	5,794,155		5,835,364
Allowance for credit losses on investment securities held-to-maturity	382	$ 308	299	$ 309	$ 312	$ 0
Net Carrying Value	5,623,243		5,567,889
Interest receivable	20,900		22,100
Agency CMO [Member]
Schedule of Investments [Line Items]
Amortized Cost(1)	61,080		91,622
Unrealized Gains	1,425		1,785
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(63)		(241)
Debt Securities, Held-to-maturity, Fair Value	62,442		93,166
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	61,080		91,622
Agency MBS [Member]
Schedule of Investments [Line Items]
Amortized Cost(1)	2,562,863		2,419,751
Unrealized Gains	96,187		137,863
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(3,684)		(84)
Debt Securities, Held-to-maturity, Fair Value	2,655,366		2,557,530
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	2,562,863		2,419,751
Agency Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost(1)	2,105,924		2,101,227
Unrealized Gains	30,491		60,484
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(16,545)		(2,213)
Debt Securities, Held-to-maturity, Fair Value	2,119,870		2,159,498
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	2,105,924		2,101,227
Municipal Bonds [Member]
Schedule of Investments [Line Items]
Amortized Cost(1)	715,195		739,507
Unrealized Gains	56,218		60,371
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	0		(3)
Debt Securities, Held-to-maturity, Fair Value	771,413		799,875
Allowance for credit losses on investment securities held-to-maturity	382		299
Net Carrying Value	714,813		739,208
Non-agency CMBS [Member]
Schedule of Investments [Line Items]
Amortized Cost(1)	178,563		216,081
Unrealized Gains	6,501		9,214
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	0		0
Debt Securities, Held-to-maturity, Fair Value	185,064		225,295
Allowance for credit losses on investment securities held-to-maturity	0		0
Net Carrying Value	$ 178,563		$ 216,081