Fair Value (Detail) - Fair Values of our Defined Benefit Pension Plan Assets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Fair Value [Line Items]
Pension plan assets	$ 72,193		$ 74,370		$ 63,501
Fair Value, Inputs, Level 1
Fair Value [Line Items]
Pension plan assets	47,527		48,418
Fair Value, Inputs, Level 2
Fair Value [Line Items]
Pension plan assets	24,666		25,952
Estimate of Fair Value Measurement
Fair Value [Line Items]
Pension plan assets	72,193		74,370
Mutual Funds | Fair Value, Inputs, Level 1
Fair Value [Line Items]
Pension plan assets	25,499	[1]	23,896	[1]
Mutual Funds | Fair Value, Inputs, Level 2
Fair Value [Line Items]
Pension plan assets	0	[1]	0	[1]
Mutual Funds | Estimate of Fair Value Measurement
Fair Value [Line Items]
Pension plan assets	25,499	[1]	23,896	[1]
U.S. Agency Securities | Fair Value, Inputs, Level 1
Fair Value [Line Items]
Pension plan assets	7,879		7,261
U.S. Agency Securities | Fair Value, Inputs, Level 2
Fair Value [Line Items]
Pension plan assets	8,063		8,930
U.S. Agency Securities | Estimate of Fair Value Measurement
Fair Value [Line Items]
Pension plan assets	15,942		16,191
Corporate Bonds | Fair Value, Inputs, Level 1
Fair Value [Line Items]
Pension plan assets	0		0
Corporate Bonds | Fair Value, Inputs, Level 2
Fair Value [Line Items]
Pension plan assets	6,599		5,984
Corporate Bonds | Estimate of Fair Value Measurement
Fair Value [Line Items]
Pension plan assets	6,599		5,984
Common Stock | Fair Value, Inputs, Level 1
Fair Value [Line Items]
Pension plan assets	14,149		17,261
Common Stock | Fair Value, Inputs, Level 2
Fair Value [Line Items]
Pension plan assets	0		0
Common Stock | Estimate of Fair Value Measurement
Fair Value [Line Items]
Pension plan assets	14,149		17,261
Common Trust Funds | Fair Value, Inputs, Level 1
Fair Value [Line Items]
Pension plan assets	0		0
Common Trust Funds | Fair Value, Inputs, Level 2
Fair Value [Line Items]
Pension plan assets	10,004		11,038
Common Trust Funds | Estimate of Fair Value Measurement
Fair Value [Line Items]
Pension plan assets	$ 10,004		$ 11,038

[1] (1) Includes mutual funds, money market funds, cash, cash equivalents, and accrued interest.