Debt Securities Available-for-Sale - Gross Unrealized Losses on Mortgage-Backed Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, less than 12 months	$ 4,772	$ 1,433
Estimated Fair value, less than 12 months	299,949	163,581
Unrealized losses, 12 months or more	12,125	8,079
Estimated Fair value, 12 months or more	210,125	231,106
Unrealized losses, total	16,897	9,512
Estimated fair value, total	510,074	394,687
Pass-through Certificates | GSE
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, less than 12 months	1,538	439
Estimated Fair value, less than 12 months	104,219	48,931
Unrealized losses, 12 months or more	3,700	2,015
Estimated Fair value, 12 months or more	74,599	76,113
Unrealized losses, total	5,238	2,454
Estimated fair value, total	178,818	125,044
Real estate mortgage investment conduits (REMICs) | GSE
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, less than 12 months	2,720	933
Estimated Fair value, less than 12 months	120,720	103,644
Unrealized losses, 12 months or more	8,040	5,926
Estimated Fair value, 12 months or more	120,653	139,830
Unrealized losses, total	10,760	6,859
Estimated fair value, total	241,373	243,474
Real estate mortgage investment conduits (REMICs) | Non-GSE
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, less than 12 months	0	0
Estimated Fair value, less than 12 months	0	0
Unrealized losses, 12 months or more	1	1
Estimated Fair value, 12 months or more	75	79
Unrealized losses, total	1	1
Estimated fair value, total	75	79
Debt securities | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, less than 12 months	514	61
Estimated Fair value, less than 12 months	75,010	11,006
Unrealized losses, 12 months or more	384	137
Estimated Fair value, 12 months or more	14,798	15,084
Unrealized losses, total	898	198
Estimated fair value, total	$ 89,808	$ 26,090