Debt Securities Available-for-Sale - Gross Unrealized Losses on Mortgage-Backed Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, less than 12 months	$ 11,053	$ 1,433
Estimated Fair value, less than 12 months	464,558	163,581
Unrealized losses, 12 months or more	8,407	8,079
Estimated Fair value, 12 months or more	159,127	231,106
Unrealized losses, total	19,460	9,512
Estimated fair value, total	623,685	394,687
Pass-through Certificates | GSE
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, less than 12 months	2,698	439
Estimated Fair value, less than 12 months	185,891	48,931
Unrealized losses, 12 months or more	3,486	2,015
Estimated Fair value, 12 months or more	65,491	76,113
Unrealized losses, total	6,184	2,454
Estimated fair value, total	251,382	125,044
Real estate mortgage investment conduits (REMICs) | GSE
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, less than 12 months	7,667	933
Estimated Fair value, less than 12 months	172,671	103,644
Unrealized losses, 12 months or more	4,564	5,926
Estimated Fair value, 12 months or more	78,771	139,830
Unrealized losses, total	12,231	6,859
Estimated fair value, total	251,442	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	60	79
Unrealized losses, total	1	1
Estimated fair value, total	60	79
Debt securities | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, less than 12 months	688	61
Estimated Fair value, less than 12 months	105,996	11,006
Unrealized losses, 12 months or more	356	137
Estimated Fair value, 12 months or more	14,805	15,084
Unrealized losses, total	1,044	198
Estimated fair value, total	$ 120,801	$ 26,090