Available-For-Sale Securities (Available-For-Sale Securities, Continuous Unrealized Loss Position, Fair Value) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	$ 693,241
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(46,831)
Continuous unrealized losses existing for greater than 12 months, Fair value	462,334
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(36,650)
Total, Fair value	1,155,575
Total, Unrealized losses	(83,481)
US Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	0
Continuous unrealized losses existing for less than 12 months, Unrealized losses	0
Continuous unrealized losses existing for greater than 12 months, Fair value	185,969
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(14,194)
Total, Fair value	185,969
Total, Unrealized losses	(14,194)
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	463,254
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(42,295)
Continuous unrealized losses existing for greater than 12 months, Fair value	54,128
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(7,561)
Total, Fair value	517,382
Total, Unrealized losses	(49,856)
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	61,280
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(2,683)
Continuous unrealized losses existing for greater than 12 months, Fair value	9,484
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(484)
Total, Fair value	70,764
Total, Unrealized losses	(3,167)
Corporate Notes, Financial Issuers [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	1,327
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(10)
Continuous unrealized losses existing for greater than 12 months, Fair value	57,474
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(1,725)
Total, Fair value	58,801
Total, Unrealized losses	(1,735)
Corporate Notes, Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	997
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(3)
Continuous unrealized losses existing for greater than 12 months, Fair value	0
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	0
Total, Fair value	997
Total, Unrealized losses	(3)
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	137,042
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(1,352)
Continuous unrealized losses existing for greater than 12 months, Fair value	137,661
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(11,836)
Total, Fair value	274,703
Total, Unrealized losses	(13,188)
Mortgage-Backed, Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	21,045
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(275)
Continuous unrealized losses existing for greater than 12 months, Fair value	11,944
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(524)
Total, Fair value	32,989
Total, Unrealized losses	(799)
Other Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	8,296
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(213)
Continuous unrealized losses existing for greater than 12 months, Fair value	5,674
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(326)
Total, Fair value	13,970
Total, Unrealized losses	$ (539)