Derivative Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	$ (731)	$ 1,334
Interest rate swaps | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(515)	7,876
Interest rate swaps | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(163)	(4,296)
Interest rate futures | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(1,380)	(430)
Interest rate futures | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	1,472	(1,177)
Futures on equity indices | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	24	(27)
Futures on equity indices | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(234)	(274)
Interest rate swaptions | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	690	9
Interest rate swaptions | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(625)	(347)
Cash flow hedges
Effect of derivative instruments in the condensed consolidated statement of income
Gain (loss) recognized in OCI on derivatives (Effective portion)	23,948	(4,546)
Gain (loss) reclassified from OCI into net income (Effective portion)	707	759
Cash flow hedges | Interest rate swaps
Effect of derivative instruments in the condensed consolidated statement of income
Gain (loss) recognized in OCI on derivatives (Effective portion)	(2,693)	508
Gain (loss) reclassified from OCI into net income (Effective portion)	691	743
Cash flow hedges | Cross-currency swaps
Effect of derivative instruments in the condensed consolidated statement of income
Gain (loss) recognized in OCI on derivatives (Effective portion)	26,641	(5,054)
Cash flow hedges | Interest rate futures
Effect of derivative instruments in the condensed consolidated statement of income
Gain (loss) reclassified from OCI into net income (Effective portion)	16	16
Fair value hedges
Effect of derivative instruments in the condensed consolidated statement of income
Gain (loss) on derivatives recognized in net income	2,276	1,139
Gain (loss) on hedged assets recognized in net income	(2,276)	(1,139)
Fair value hedges | Interest rate swaps | Net investment income
Effect of derivative instruments in the condensed consolidated statement of income
Gain (loss) on derivatives recognized in net income	1,892	1,139
Gain (loss) on hedged assets recognized in net income	(1,892)	(1,139)
Fair value hedges | Interest rate swaps | Realized investment gains (losses)
Effect of derivative instruments in the condensed consolidated statement of income
Gain (loss) on derivatives recognized in net income	384
Gain (loss) on hedged assets recognized in net income	$ (384)