Investment Securities (Contractual Maturities Debt Securities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investments [Abstract]
Held-to-maturity, Due in one year or less, Amortized cost	$ 49,885
Held-to-maturity, Due in one year or less, Estimated fair value	50,155
Held-to-maturity, Due after one year through five years, Amortized cost	201,851
Held-to-maturity, Due after one year through five years, Estimated fair value	196,829
Held-to-maturity, Due after five years through ten years, Amortized cost	170,597
Held-to-maturity, Due after five years through ten years, Estimated fair value	148,633
Held-to-maturity, Due after ten years, Amortized cost	374,216
Held-to-maturity, Due after ten years, Estimated fair value	260,151
Held-to-maturity, Amortized cost	796,549	851,731
Held-to-maturity, Estimated fair value	655,768
Available-for-sale, Due in one year or less, Amortized cost	450,217
Available-for-sale, Due in one year or less, Estimated fair value	416,979
Available-for-sale, Due after one year through five years, Amortized cost	1,032,915
Available-for-sale, Due after one year through five years, Estimated fair value	958,822
Available-for-sale, Due after five years through ten years, Amortized cost	690,633
Available-for-sale, Due after five years through ten years, Estimated fair value	597,124
Available-for-sale, Due after ten years, Amortized cost	1,491,078
Available-for-sale, Due after ten years, Estimated fair value	936,726
Available-for-sale, Amortized cost	3,664,843
Available-for-sale, Estimated fair value	$ 2,909,651