Investment Securities (Contractual Maturities Debt Securities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Investments [Abstract]
Held-to-maturity, Due in one year or less, Amortized cost	$ 54,912
Held-to-maturity, Due in one year or less, Estimated fair value	55,321
Held-to-maturity, Due after one year through five years, Amortized cost	207,047
Held-to-maturity, Due after one year through five years, Estimated fair value	201,654
Held-to-maturity, Due after five years through ten years, Amortized cost	172,813
Held-to-maturity, Due after five years through ten years, Estimated fair value	154,922
Held-to-maturity, Due after ten years, Amortized cost	394,589
Held-to-maturity, Due after ten years, Estimated fair value	303,813
Held-to-maturity, Amortized cost	829,361	851,731
Held-to-maturity, Estimated fair value	715,710
Available-for-sale, Due in one year or less, Amortized cost	450,929
Available-for-sale, Due in one year or less, Estimated fair value	420,393
Available-for-sale, Due after one year through five years, Amortized cost	1,108,393
Available-for-sale, Due after one year through five years, Estimated fair value	1,006,558
Available-for-sale, Due after five years through ten years, Amortized cost	690,377
Available-for-sale, Due after five years through ten years, Estimated fair value	602,812
Available-for-sale, Due after ten years, Amortized cost	1,527,057
Available-for-sale, Due after ten years, Estimated fair value	924,851
Available-for-sale, Amortized cost	3,776,756
Available-for-sale, Estimated fair value	$ 2,954,614