Investment Securities (Contractual Maturities Debt Securities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investments [Abstract]
Held-to-maturity, Due in one year or less, Amortized cost	$ 66,525
Held-to-maturity, Due in one year or less, Estimated fair value	66,502
Held-to-maturity, Due after one year through five years, Amortized cost	200,748
Held-to-maturity, Due after one year through five years, Estimated fair value	198,494
Held-to-maturity, Due after five years through ten years, Amortized cost	213,234
Held-to-maturity, Due after five years through ten years, Estimated fair value	179,794
Held-to-maturity, Due after ten years, Amortized cost	353,947
Held-to-maturity, Due after ten years, Estimated fair value	283,118
Held-to-maturity, Amortized cost	834,454	802,343
Held-to-maturity, fair value	727,908	674,741
Available-for-sale, Due in one year or less, Amortized cost	466,546
Available-for-sale, Due in one year or less, Estimated fair value	441,052
Available-for-sale, Due after one year through five years, Amortized cost	1,112,538
Available-for-sale, Due after one year through five years, Estimated fair value	1,050,208
Available-for-sale, Due after five years through ten years, Amortized cost	640,232
Available-for-sale, Due after five years through ten years, Estimated fair value	597,543
Available-for-sale, Due after ten years, Amortized cost	1,359,652
Available-for-sale, Due after ten years, Estimated fair value	988,055
Available-for-sale, Amortized cost	3,578,968
Available-for-sale, Estimated fair value	$ 3,076,858	$ 2,863,299