Reverse Repurchase Agreements / Securities Lending Transactions - Details surrounding amounts reclassified out of accumulated other comprehensive income (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Summary of amounts outstanding under reverse repurchase agreements
Gross Amount Of Recognized Asset	$ 332,602	$ 375,658	$ 406,073
Gross Amount Of Recognized Liabilities	264,556	327,543	173,599
Gross Amount Offset on the Balance Sheet - Asset	(250,000)	(315,000)	(171,000)
Gross Amount Offset on the Balance Sheet - Liabilities	(250,000)	(315,000)	(171,000)
Net Asset Balance	82,602	60,658	235,073
Net Liability Balance	14,556	12,543	2,599
Weighted average interest rate of reverse repurchase agreements - Assets	1.28%	1.18%	1.17%
Weighted average interest rate of reverse repurchase agreements - Liabilities	0.37%	0.43%	0.32%
Gross Amounts not Offset in the Balance Sheet - Financial Instrument - Asset
Gross Amounts not Offset in the Balance Sheet - Financial Instrument - Liabilities
Collateral Received
Collateral Pledged	14,585	11,493	2,919
Net Amount Asset	82,602	60,658	235,073
Net Amount Liabilities		1,050
Repurchase agreements / reverse repurchase agreements
Summary of amounts outstanding under reverse repurchase agreements
Gross Amount Of Recognized Asset	332,000	325,000	406,000
Gross Amount Of Recognized Liabilities	250,000	265,000	171,000
Gross Amount Offset on the Balance Sheet - Asset	(250,000)	(265,000)	(171,000)
Gross Amount Offset on the Balance Sheet - Liabilities	(250,000)	(265,000)	(171,000)
Net Asset Balance	82,000	60,000	235,000
Net Liability Balance
Gross Amounts not Offset in the Balance Sheet - Financial Instrument - Asset
Gross Amounts not Offset in the Balance Sheet - Financial Instrument - Liabilities
Collateral Received
Collateral Pledged
Net Amount Asset	82,000	60,000	235,000
Net Amount Liabilities
Securities lending transactions
Summary of amounts outstanding under reverse repurchase agreements
Gross Amount Of Recognized Asset		50,000
Gross Amount Of Recognized Liabilities		50,000
Gross Amount Offset on the Balance Sheet - Asset		(50,000)
Gross Amount Offset on the Balance Sheet - Liabilities		(50,000)
Net Asset Balance
Net Liability Balance
Gross Amounts not Offset in the Balance Sheet - Financial Instrument - Asset
Gross Amounts not Offset in the Balance Sheet - Financial Instrument - Liabilities
Collateral Received
Collateral Pledged
Net Amount Asset
Net Amount Liabilities
Derivatives
Summary of amounts outstanding under reverse repurchase agreements
Gross Amount Of Recognized Asset	602	658	73
Gross Amount Of Recognized Liabilities	14,556	12,543	2,599
Gross Amount Offset on the Balance Sheet - Asset
Gross Amount Offset on the Balance Sheet - Liabilities
Net Asset Balance	602	658	73
Net Liability Balance	14,556	12,543	2,599
Gross Amounts not Offset in the Balance Sheet - Financial Instrument - Asset
Gross Amounts not Offset in the Balance Sheet - Financial Instrument - Liabilities
Collateral Received
Collateral Pledged	14,585	11,493	2,919
Net Amount Asset	602	658	73
Net Amount Liabilities		$ 1,050