Available-For-Sale Securities (Available-For-Sale Securities, Continuous Unrealized Loss Position, Fair Value) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	$ 432,543
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(4,630)
Continuous unrealized losses existing for greater than 12 months, Fair value	488,458
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(11,848)
Total, Fair value	921,001
Total, Unrealized losses	(16,478)
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	198,297
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(1,847)
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	198,297
Total, Unrealized losses	(1,847)
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	163,928
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(2,158)
Continuous unrealized losses existing for greater than 12 months, Fair value	259,346
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(6,574)
Total, Fair value	423,274
Total, Unrealized losses	(8,732)
Municipal
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	41,611
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(500)
Continuous unrealized losses existing for greater than 12 months, Fair value	37,899
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(1,022)
Total, Fair value	79,510
Total, Unrealized losses	(1,522)
Corporate notes, Financial issuers
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	9,968
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(31)
Continuous unrealized losses existing for greater than 12 months, Fair value	44,667
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(1,290)
Total, Fair value	54,635
Total, Unrealized losses	(1,321)
Corporate notes, Other
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	999
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(1)
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	999
Total, Unrealized losses	(1)
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	16,725
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(92)
Continuous unrealized losses existing for greater than 12 months, Fair value	127,433
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(2,437)
Total, Fair value	144,158
Total, Unrealized losses	(2,529)
Mortgage-backed, Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	1,015
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(1)
Continuous unrealized losses existing for greater than 12 months, Fair value	10,502
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(275)
Total, Fair value	11,517
Total, Unrealized losses	(276)
Equity securities
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	8,611
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(250)
Total, Fair value	8,611
Total, Unrealized losses	$ (250)