Summary of Significant Accounting Policies - Computation of Basic EPS to Dilutive EPS (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Regulatory Assets [Abstract]
Net earnings	$ 89,559	$ 78,868	$ 74,225
Effect of stock options, Net Earnings	0	0	0
Net earnings per share, assuming dilution, Net Earnings	$ 89,559	$ 78,868	$ 74,225
Net earnings per share, basic, Weighted Average Shares	64,047,803	63,575,948	62,960,310
Effect of stock options, Weighted Average Shares	260,732	280,816	42,224
Net earnings per share, assuming dilution, Weighted Average Shares	64,308,535	63,856,764	63,002,534
Net earnings per share, basic, Per Share Amount	$ 1.40	$ 1.24	$ 1.18
Effect of stock options, Per Share Amount	$ (0.01)
Net earnings per share, assuming dilution, Per Share Amount	$ 1.39	$ 1.24	$ 1.18