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Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Available for sale ("AFS") securities:
Income before income taxes	$ 85,515	$ 56,363	$ 165,120	$ 115,388
Related income tax effect	(19,241)	(19,304)	(37,140)	(39,520)
Net income	66,274	37,059	127,980	75,868
Pension plan:
Income before income taxes	85,515	56,363	165,120	115,388
Related income tax effect	(19,241)	(19,304)	(37,140)	(39,520)
Net income	$ 66,274	$ 37,059	127,980	$ 75,868
Reclassification out of Accumulated Other Comprehensive Income [Member]
Available for sale ("AFS") securities:
Net income			1,920
Pension plan:
Net income			1,920
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			176
Income before income taxes			176
Related income tax effect			(41)
Net income			135
Pension plan:
Income before income taxes			176
Related income tax effect			(41)
Net income			135
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Accumulated Defined Benefit Plans Adjustment [Member]
Available for sale ("AFS") securities:
Income before income taxes			2,311
Related income tax effect			(526)
Net income			1,785
Pension plan:
Recognized net actuarial loss			2,311
Income before income taxes			2,311
Related income tax effect			(526)
Net income			$ 1,785