Fair Values of Financial and Nonfinancial Instruments (Details) (USD $)	Mar. 31, 2013 Loan	Dec. 31, 2012 Loan
Fair value measurement for financial instruments
Marketable securities, available-for-sale	$ 363,058,000	$ 388,020,000
Mortgage loans held-for-sale, unpaid principal balance	55,800,000	103,400,000
Mortgage loans held-for-sale, difference between the aggregate fair value and the aggregate unpaid principal balance	1,800,000	4,600,000
Number of loans 90 days or more past due	2	0
Mortgage loans held-for-sale, aggregate carrying value of loans with payments 90 days or more past due	374,000
Mortgage loans held-for-sale, aggregate unpaid principal balance of loans with payments 90 days or more past due	526,000
U.S. Treasury securities
Fair value measurement for financial instruments
Marketable securities, available-for-sale	29,487,000	3,099,000
Obligations of U.S. and local government agencies
Fair value measurement for financial instruments
Marketable securities, available-for-sale	92,433,000	155,293,000
Corporate debt securities
Fair value measurement for financial instruments
Marketable securities, available-for-sale	197,033,000	165,194,000
Asset-backed securities
Fair value measurement for financial instruments
Marketable securities, available-for-sale	38,584,000	27,314,000
Short-term pooled investments
Fair value measurement for financial instruments
Marketable securities, available-for-sale	5,521,000	37,120,000
Recurring Measurement \| Levels 1 and 2 \| Obligations of U.S. and local government agencies
Fair value measurement for financial instruments
Fair value of assets	92,433,000	155,293,000
Recurring Measurement \| Levels 1 and 2 \| Short-term pooled investments
Fair value measurement for financial instruments
Fair value of assets	5,521,000	37,120,000
Recurring Measurement \| Level 1 \| U.S. Treasury securities
Fair value measurement for financial instruments
Fair value of assets	29,487,000	3,099,000
Recurring Measurement \| Level 2 \| Mortgage loans held-for-sale
Fair value measurement for financial instruments
Fair value of assets	57,568,000	107,950,000
Recurring Measurement \| Level 2 \| Mortgage interest rate lock commitments ("IRLCs")
Fair value measurement for financial instruments
Fair value of assets	6,084,000	4,737,000
Recurring Measurement \| Level 2 \| Forward-delivery contracts
Fair value measurement for financial instruments
Fair value of assets	(513,000)	(369,000)
Recurring Measurement \| Level 2 \| Corporate debt securities
Fair value measurement for financial instruments
Fair value of assets	197,033,000	165,194,000
Recurring Measurement \| Level 2 \| Asset-backed securities
Fair value measurement for financial instruments
Fair value of assets	$ 38,584,000	$ 27,314,000

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Number 115

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15C

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

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Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Number FAS115-1/124-1

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Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 14

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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