Employee Benefits - Changes in Level 3 Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Balance at end of year	$ 33,812	$ 32,322
Fair Value Measurements, Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance at beginning of year	2,256	1,691
Purchases	32	55
Redemptions	(58)
Actual return - assets still being held	130	510
Balance at end of year	$ 2,360	$ 2,256