Fair Value Measurements (Narrative) (Detail) $ in Thousands	Dec. 31, 2017 USD ($) Grade	Dec. 31, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments using fair value measurement option | $	$ 0	$ 0
Commodity Swaps And Options [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average risk grade of loans underlying swaps measured at fair value using significant unobservable inputs | Grade	12.0
Weighted average loss severity on default of swaps	15.40%