Employee Benefit Plans - Changes in the Benefit Obligation (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Executive Retirement Health Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated postretirement benefit obligation, beginning		$ 2,674	$ 3,574
Service cost		174	649	919
Interest cost		78	134	150
Actuarial experience and changes in actuarial assumptions		234	(834)
Curtailment	(785)
Settlement/curtailment loss/(gain)		725	(785)
Benefits Paid		(3,885)	(64)
Accumulated postretirement benefit obligation, ending	2,674	0	2,674	3,574
Supplemental Executive Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated postretirement benefit obligation, beginning		7,852	19,658
Service cost		1,164	2,018	2,087
Interest cost		294	471	560
Actuarial experience and changes in actuarial assumptions		887	(1,475)
Curtailment	(2,100)	0	(2,138)
Benefits Paid		0	(10,682)
Accumulated postretirement benefit obligation, ending	$ 7,852	$ 10,197	$ 7,852	$ 19,658