Investment Securities (Contractual Maturities Debt Securities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments [Abstract]
Held-to-maturity, Due in one year or less, Amortized cost	$ 63,400
Held-to-maturity, Due in one year or less, Estimated fair value	63,302
Held-to-maturity, Due after one year through five years, Amortized cost	192,533
Held-to-maturity, Due after one year through five years, Estimated fair value	187,481
Held-to-maturity, Due after five years through ten years, Amortized cost	216,669
Held-to-maturity, Due after five years through ten years, Estimated fair value	188,033
Held-to-maturity, Due after ten years, Amortized cost	368,044
Held-to-maturity, Due after ten years, Estimated fair value	295,476
Held-to-maturity, Amortized cost	840,646	802,343
Held-to-maturity, fair value	734,292	674,741
Available-for-sale, Due in one year or less, Amortized cost	436,513
Available-for-sale, Due in one year or less, Estimated fair value	415,076
Available-for-sale, Due after one year through five years, Amortized cost	1,032,220
Available-for-sale, Due after one year through five years, Estimated fair value	972,291
Available-for-sale, Due after five years through ten years, Amortized cost	615,276
Available-for-sale, Due after five years through ten years, Estimated fair value	564,084
Available-for-sale, Due after ten years, Amortized cost	1,329,643
Available-for-sale, Due after ten years, Estimated fair value	973,375
Available-for-sale, Amortized cost	3,413,652
Available-for-sale, Estimated fair value	$ 2,924,826	$ 2,863,299