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Revenue Recognition - Schedule of Impact of Adopting New Revenue Standard on Company's Unaudited Condensed Consolidated Statement of Income (Detail) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Statement of Income    
Operating revenues $ 351,316 $ 301,207
Operating income 167,166 130,732
Income before provision for income taxes 139,438 101,625
Provision for income taxes 24,346 28,674
Net income $ 115,092 $ 72,951
Earnings per basic common share $ 1.28 $ 0.80
Earnings per diluted common share $ 1.24 $ 0.80
Accounting Standards Update 2014-09 [Member] | Impact of Change [Member]    
Statement of Income    
Operating revenues $ (2,286)  
Operating income (2,286)  
Income before provision for income taxes (2,286)  
Provision for income taxes 457  
Net income $ (1,829)  
Earnings per basic common share $ (0.02)  
Earnings per diluted common share $ (0.02)  
Accounting Standards Update 2014-09 [Member] | Without Adoption of ASC 606 [Member]    
Statement of Income    
Operating revenues $ 349,030  
Operating income 164,880  
Income before provision for income taxes 137,152  
Provision for income taxes 24,803  
Net income $ 113,263  
Earnings per basic common share $ 1.26  
Earnings per diluted common share $ 1.22