Securities Available for Sale - Maturities (Details) $ in Thousands	Sep. 30, 2015 USD ($)
Fair value
Within 1 year	$ 28,120
After 1 year through 5 years	310,399
After 5 years through 10 years	0
After 10 years	0
Total	338,519
Amortized cost
Within 1 year	28,151
After 1 year through 5 years	311,540
After 5 years through 10 years	0
After 10 years	0
Total	339,691
Corporate debt securities
Fair value
Within 1 year	28,120
After 1 year through 5 years	221,993
After 5 years through 10 years	0
After 10 years	0
Total	250,113
Asset-backed securities
Fair value
Within 1 year	0
After 1 year through 5 years	54,959
After 5 years through 10 years	0
After 10 years	0
Total	54,959
U.S. agency securities
Fair value
Within 1 year	0
After 1 year through 5 years	20,603
After 5 years through 10 years	0
After 10 years	0
Total	20,603
Municipal securities
Fair value
Within 1 year	0
After 1 year through 5 years	5,843
After 5 years through 10 years	0
After 10 years	0
Total	5,843
Other securities
Fair value
Within 1 year	0
After 1 year through 5 years	7,001
After 5 years through 10 years	0
After 10 years	0
Total	$ 7,001