Available-For-Sale Securities (Available-For-Sale Securities, Continuous Unrealized Loss Position, Fair Value) (Detail) $ in Thousands	Sep. 30, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	$ 1,043,038
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(15,500)
Continuous unrealized losses existing for greater than 12 months, Fair value	471,988
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(16,748)
Total, Fair value	1,515,026
Total, Unrealized losses	(32,248)
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	202,795
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(2,364)
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	202,795
Total, Unrealized losses	(2,364)
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	218,297
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(4,932)
Continuous unrealized losses existing for greater than 12 months, Fair value	251,654
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(10,068)
Total, Fair value	469,951
Total, Unrealized losses	(15,000)
Municipal
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	68,140
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(964)
Continuous unrealized losses existing for greater than 12 months, Fair value	37,002
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(1,121)
Total, Fair value	105,142
Total, Unrealized losses	(2,085)
Corporate notes, Financial issuers
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	23,052
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(152)
Continuous unrealized losses existing for greater than 12 months, Fair value	44,894
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(1,064)
Total, Fair value	67,946
Total, Unrealized losses	(1,216)
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	(2)
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	(2)
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	519,631
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(7,011)
Continuous unrealized losses existing for greater than 12 months, Fair value	120,261
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(3,902)
Total, Fair value	639,892
Total, Unrealized losses	(10,913)
Mortgage-backed, Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	7,167
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(63)
Continuous unrealized losses existing for greater than 12 months, Fair value	9,620
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(260)
Total, Fair value	16,787
Total, Unrealized losses	(323)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Continuous unrealized losses existing for less than 12 months, Fair value	2,957
Continuous unrealized losses existing for less than 12 months, Unrealized losses	(12)
Continuous unrealized losses existing for greater than 12 months, Fair value	8,557
Continuous unrealized losses existing for greater than 12 months, Unrealized losses	(333)
Total, Fair value	11,514
Total, Unrealized losses	$ (345)