Pension Plan - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum required pension plan contribution	$ 5,100,000		$ 5,100,000
Voluntary contribution			56,900,000
Available credit balance	2,500,000		2,500,000
Net periodic pension income	310,000	$ 641,000	616,000	$ 1,275,000
Expected return on plan assets	2,390,000	1,907,000	4,753,000	3,793,000
Interest cost on projected benefit obligation	1,228,000	$ 1,096,000	2,442,000	$ 2,180,000
Pentegra Defined Benefit Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum required pension plan contribution	0		0
Voluntary contribution	200,000
Spinning off of asset cost	2,800,000
Net periodic pension income	62,000		100,000
Expected return on plan assets	200,000		300,000
Recognized net actuarial gain	3,000		6,000
Interest cost on projected benefit obligation	$ 100,000		$ 200,000