Summary of Effective Income Tax Rate (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of Effective Income Tax Rate [Line Items]
Expected income taxes	$ 268,098	$ 253,324	$ 262,700
Tax-exempt investment income	(3,506)	(3,468)	(3,371)
Low income housing investments	(28,877)	(24,258)	(12,900)
Other	954	568	46
Income tax expense	$ 236,669	$ 226,166	$ 246,475
Expected income taxes	35.00%	35.00%	35.00%
Tax-exempt investment income	(0.40%)	(0.50%)	(0.50%)
Low income housing investments	(3.80%)	(3.40%)	(1.70%)
Other	0.10%	0.10%	0.00%
Income tax expense	30.90%	31.20%	32.80%