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Comprehensive Income - Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Amounts Reclassified Out Of Accumulated Other Comprehensive Income Loss [Abstract] |  |
Unrealized Gains (Losses) on Available-for-Sale Securities, Net realized investment gains | $ 6,082 |
Unrealized Gains (Losses) on Available-for-Sale Securities, Tax (expense) benefit |  |
Unrealized Gains (Losses) on Available-for-Sale Securities, Net of tax and noncontrolling interest | 6,082 |
Amortization of Postretirement Benefit, Prior service cost | (1,707) |
Amortization of Postretirement Benefit, Actuarial gains (losses) | 899 |
Amortization of Postretirement Benefit, Total before tax | (808) |
Amortization of Postretirement, Tax (expense) benefit |  |
Amortization of Postretirement, Net of tax and noncontrolling interest | (808) | 3,792
Total reclassfications for the period | $ 5,274 |

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- Definition The pretax net amount of gains and losses that are not yet recognized as a component of net periodic benefit cost, and that are recognized as increases or decreases in other comprehensive income as they arise. Gains and losses are due to changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption, or the consequence of a decision to temporarily deviate from the substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=20435746&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 158 -Paragraph 7 -Subparagraph c -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph i -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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 + Details
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