Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statement Of Partners Capital [Abstract]
Net income	$ 59,663	$ 58,062	$ 114,851	$ 119,095
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss during the period	(74,025)	20,968	(95,369)	22,850
Change in net unrealized gain on securities transferred to held to maturity	(9,745)		(18,204)
Reclassification adjustment for net (gains) losses included in net income	(6)	(370)	(11)	121
Total securities available for sale and transferred securities	(83,776)	20,598	(113,584)	22,971
Defined-benefit post-retirement benefit plans:
Change in the net actuarial gain/loss	1,639	1,428	3,279	2,657
Derivatives:
Change in the accumulated gain/loss on effective cash flow hedge derivatives	(33)	(64)	(33)	(491)
Reclassification adjustments for (gains) losses included in net income:
Interest rate swaps on variable-rate loans	(9,345)	(9,345)	(18,690)	(18,690)
Interest rate swap on junior subordinated deferrable interest debentures	1,103	1,044	2,188	2,077
Total derivatives	(8,275)	(8,365)	(16,535)	(17,104)
Other comprehensive income (loss), before tax	(90,412)	13,661	(126,840)	8,524
Deferred tax expense (benefit) related to other comprehensive income	(31,644)	4,782	(44,394)	2,983
Other comprehensive income (loss), net of tax	(58,768)	8,879	(82,446)	5,541
Comprehensive income	$ 895	$ 66,941	$ 32,405	$ 124,636