Fair Value Measurements - Fair Value Methods Used for Measuring Fair Values of Financial Instruments on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Total available-for-sale securities	$ 634,441		$ 551,938
Metropolitan district bond securities (available-for-sale)	13,835		5,818
Level 1 [Member]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Equity securities	368,579		208,818
Level 2 [Member]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Debt securities - maturity less than 1 year	111,397		54,388
Debt securities - maturity 1 to 5 years	138,606		277,514
Debt securities - maturity greater than 5 years	15,859		11,218
Mortgage loans held-for-sale, net	92,463		119,953
Metropolitan district bond securities [Member]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Metropolitan district bond securities (available-for-sale)	13,835	5,818	5,818	6,663	6,663	6,663
Metropolitan district bond securities [Member] | Level 3 [Member]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities [Line Items]
Metropolitan district bond securities (available-for-sale)	$ 13,835		$ 5,818