Derivative Instruments, Outstanding Notional Amounts and Interest Rate Swap Agreements of Derivative Instruments (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2019	Dec. 31, 2018
Notional Amount of Interest Rate Swaps [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 2,404,350	$ 1,380,000
Weighted average pay rate		1.86%	2.18%
Weighted average receive rate		2.09%	2.61%
Weighted average years to maturity		5 years 7 months 6 days	5 years 1 month 6 days
Notional Amount of Swaptions [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 35,000	$ 110,000
Weighted average pay rate		3.08%	3.25%
Weighted average years to maturity		10 years 6 months	10 years
Notional Amount of Swaptions [Member] | LIBOR [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Weighted average receive rate type	[1]	LIBOR-BBA%	LIBOR-BBA%
Not Designated as Hedging Instrument [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 3,095,050	$ 1,605,000
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		2,404,350	1,380,000
Not Designated as Hedging Instrument [Member] | Notional Amount of Swaptions [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		35,000	110,000
Not Designated as Hedging Instrument [Member] | Notional Amount of TBAs, Net [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		244,700	35,000
Not Designated as Hedging Instrument [Member] | Notional Amount of Treasury Futures [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 411,000	$ 80,000

[1] Floats in accordance with LIBOR.