Investment securities HTM Contractual Maturity (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Amortized cost and fair values of investment securities held to maturity
Securities held to maturity, debt securities due in one year or less, amortized cost	$ 0
Securities held to maturity, debt securities due after one year through five years, amortized cost	2,264
Securities held to maturity, debt securities due after five years through ten years, amortized cost	120,085
Securities held to maturity, debt securities due after ten years, amortized cost	282,106
Total held to maturity debt securities, amortized Cost	404,455us-gaap_HeldToMaturitySecurities	175,210us-gaap_HeldToMaturitySecurities
Securities held to maturity, debt securities due in one year or less, fair value	0
Securities held to maturity, debt securities due in one year through five years, fair value	2,285
Securities held to maturity, debt securities due after five years through ten years, fair value	121,721
Securities held to maturity, debt securities due after ten years, fair value	289,061
Investment securities held to maturity, fair value	$ 413,067	$ 169,889
Ratio of Fair Value to Amortized Cost, HTM	102.10%asb_Ratiooffairvaluetoamortizedcosthtm

Ratio of Fair Value to Amortized Cost, HTM

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount of held-to-maturity debt securities at fair value maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the fifth through tenth fiscal year following the latest fiscal year.

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Amount of held-to-maturity debt securities at fair value maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year.

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Amount of held-to-maturity debt securities at fair value maturing after the tenth fiscal year following the latest fiscal year.

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Amount of held-to-maturity debt securities at fair value maturing in the next fiscal year following the latest fiscal year.

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Fair value of investments in debt securities classified as held-to-maturity.

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