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Other Comprehensive Income (Loss) (Components of AOCI) (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]        
Beginning balance     $ 1,251,781 $ 1,196,065
Current period other comprehensive income (loss) $ 569 $ (1,144) 2,689 8,520
Ending Balance 1,300,172 1,244,280 1,300,172 1,244,280
Unrealized gains on securities available for sale [Member]        
AOCI Attributable to Parent, Net of Tax [Roll Forward]        
Beginning balance 1,489 13,967 (510) 3,951
Current period other comprehensive income (loss) 479 (1,515) 2,478 8,501
Ending Balance 1,968 12,452 1,968 12,452
Post-retirement obligations [Member]        
AOCI Attributable to Parent, Net of Tax [Roll Forward]        
Beginning balance (2,987) (6,185) (3,056) (6,424)
Current period other comprehensive income (loss) 36 141 105 380
Ending Balance (2,951) (6,044) (2,951) (6,044)
Unrealized gains (losses) on derivatives (cash flow hedges) [Member]        
AOCI Attributable to Parent, Net of Tax [Roll Forward]        
Beginning balance 221 (664) 169 (73)
Current period other comprehensive income (loss) 54 230 106 (361)
Ending Balance 275 (434) 275 (434)
Accumulated Other Comprehensive Income (Loss) [Member]        
AOCI Attributable to Parent, Net of Tax [Roll Forward]        
Beginning balance (1,277) 7,118 (3,397) (2,546)
Current period other comprehensive income (loss) 569 (1,144) 2,689 8,520
Ending Balance $ (708) $ 5,974 $ (708) $ 5,974