Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Available for sale ("AFS") securities:
Net reclassification adjustment for losses (gains) included in net income	$ 467	$ 1	$ 5,214	$ 251
Income before income taxes	84,574	60,325	199,962	161,080
Related income tax effect	(27,836)	(18,846)	(67,356)	(53,103)
Net income	56,738	41,479	132,606	107,977
Pension plan:
Recognized net actuarial loss	1,111	1,161	3,298	3,458
Income before income taxes	84,574	60,325	199,962	161,080
Related income tax effect	(27,836)	(18,846)	(67,356)	(53,103)
Net income	$ 56,738	$ 41,479	132,606	$ 107,977
Reclassification out of Accumulated Other Comprehensive Income [Member]
Available for sale ("AFS") securities:
Net income			1,197
Pension plan:
Net income			1,197
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains/Losses on AFS Securities [Member]
Available for sale ("AFS") securities:
Reclassification of previous noncredit OTTI to credit OTTI			0
Net reclassification adjustment for losses (gains) included in net income			(1,444)
Income before income taxes			(1,444)
Related income tax effect			534
Net income			(910)
Pension plan:
Income before income taxes			(1,444)
Related income tax effect			534
Net income			(910)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Available for sale ("AFS") securities:
Income before income taxes			3,298
Related income tax effect			(1,191)
Net income			2,107
Pension plan:
Recognized net actuarial loss			3,298
Income before income taxes			3,298
Related income tax effect			(1,191)
Net income			$ 2,107