Employer Sponsored Benefit Plan (Trust's Assets at Fair Value) (Detail) - Domestic Pension Plan - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	$ 37,038	$ 35,413	$ 33,275
Measured at NAV	14,362
Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	10,964	14,789
International equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	10,944	9,383
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	10,499	9,949
Measured at NAV	10,499
Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	3,863
Measured at NAV	3,863
Real estate investment trusts
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value		195
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	768	1,097
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	22,676	35,413
Level 1 | Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	10,964	14,789
Level 1 | International equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	10,944	9,383
Level 1 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	9,949
Level 1 | Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0
Level 1 | Real estate investment trusts
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value		195
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	768	1,097
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 2 | Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 2 | International equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 2 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 2 | Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0
Level 2 | Real estate investment trusts
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value		0
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3 | Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3 | International equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3 | Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0
Level 3 | Real estate investment trusts
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value		0
Level 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	$ 0	$ 0