Derivative Instruments, Outstanding Notional Amounts and Interest Rate Swap Agreements of Derivative Instruments (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Notional Amount of Interest Rate Swaps [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 1,380,000	$ 1,067,950
Weighted average pay rate		2.18%	1.83%
Weighted average receive rate		2.61%	1.44%
Weighted average years to maturity		5 years	4 years 10 months 24 days
Notional Amount of Swaptions [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 110,000	$ 155,000
Weighted average pay rate		3.25%	2.88%
Weighted average years to maturity		10 years	10 years 9 months 18 days
Notional Amount of Swaptions [Member] | LIBOR-BBA [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Weighted average receive rate type	[1]	LIBOR-BBA%
Not Designated as Hedging Instrument [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		$ 1,605,000	$ 1,249,850
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		1,380,000	1,067,950
Not Designated as Hedging Instrument [Member] | Notional Amount of Swaptions [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		110,000	155,000
Not Designated as Hedging Instrument [Member] | Notional Amount of TBAs, Net [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		35,000	26,900
Not Designated as Hedging Instrument [Member] | Notional Amount of Treasury Futures [Member]
Outstanding Notional Amounts and Interest Rate Swap Agreements [Abstract]
Total notional amount		80,000	0
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		$ 0	$ 0

[1] Floats in accordance with LIBOR.