Derivative Instruments, Summary of Outstanding Notional Amounts and Interest Rate Swap Agreements of Derivative Instruments (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Total notional amount		$ 80,000	$ 70,000
Weighted average pay rate		2.76%	2.74%
Weighted average receive rate type	[1]	LIBOR-BBA%	LIBOR-BBA%
Weighted average years to maturity		10 years 10 months 24 days	10 years 10 months 24 days
Notional Amount of Interest Rate Swaps [Member]
Derivative [Line Items]
Total notional amount		$ 609,950	$ 415,850
Weighted average pay rate		1.70%	1.46%
Weighted average receive rate		1.07%	0.90%
Weighted average years to maturity		5 years 6 months	4 years 9 months 18 days
Notional Amount of Options on Treasury Futures [Member]
Derivative [Line Items]
Total notional amount		$ 15,000	$ 20,000
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Total notional amount		742,450	549,850
Not Designated as Hedging Instrument [Member] | Notional Amount of Interest Rate Swaps [Member]
Derivative [Line Items]
Total notional amount		609,950	415,850
Not Designated as Hedging Instrument [Member] | Notional Amount of Swaptions [Member]
Derivative [Line Items]
Total notional amount		80,000	70,000
Not Designated as Hedging Instrument [Member] | Notional Amount of TBAs, Net [Member]
Derivative [Line Items]
Total notional amount		0
Not Designated as Hedging Instrument [Member] | Notional Amount of TBAs, Net [Member] | Short [Member]
Derivative [Line Items]
Total notional amount			6,000
Not Designated as Hedging Instrument [Member] | Notional Amount of Treasury Futures [Member]
Derivative [Line Items]
Total notional amount		$ 37,500	$ 50,000

[1] Floats in accordance with LIBOR.