Derivative Instruments, Outstanding Notional Amounts and Interest Rate Swap Agreements of Derivative Instruments (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2021	Dec. 31, 2020
Notional Amount of Interest Rate Swaps [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 1,427,500	$ 1,451,900
Fair value		$ 14,268	$ 4,913
Weighted average pay rate		0.50%	0.45%
Weighted average receive rate		0.72%	0.84%
Weighted average years to maturity		6 years 7 months 6 days	6 years 4 months 24 days
Notional Amount of Swaptions [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 70,000	$ 70,000
Fair value		$ 1,516	$ 798
Weighted average pay rate		1.54%	1.32%
Weighted average years to maturity	[1]	9 years 3 months 18 days	10 years 7 months 6 days
Weighted average years to expiration		7 months 6 days	6 months
Notional Amount of Swaptions [Member] | LIBOR [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Weighted average receive rate type	[2]	LIBOR-BBA%
Notional Amount of TBAs, Net [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 352,900	$ 332,000
Fair value		362,892	343,849
Notional Amount of U.S. Treasury Futures [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Fair value		(24)	252
Notional Amount of Options on Treasury Futures [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Fair value		83
Not Designated as Hedging Instrument [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		1,947,400	1,963,900
Not Designated as Hedging Instrument [Member] | Notional Amount of Interest Rate Swaps [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		1,427,500	1,451,900
Not Designated as Hedging Instrument [Member] | Notional Amount of Swaptions [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		70,000	70,000
Not Designated as Hedging Instrument [Member] | Notional Amount of TBAs, Net [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		352,900	332,000
Not Designated as Hedging Instrument [Member] | Notional Amount of U.S. Treasury Futures [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		67,000	110,000
Not Designated as Hedging Instrument [Member] | Notional Amount of Options on Treasury Futures [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 30,000	$ 0

[1] Weighted average years to maturity of the underlying swaps from the reporting date.

[2] Floats in accordance with LIBOR.